(MONEY Magazine) – BRAIN

• WHAT TO EXPECT By your mid-forties, 9,000 brain cells a day are dying. (Don't panic: You still have 100 billion.) Some memory loss and slowing of reflexes is normal. Alzheimer's disease isn't a threat for most men and women for a couple more decades, and there is as yet no way to detect it this far in advance. But researchers hope to find ways to use MRIs and PET scans to screen fortysomethings for early warning signs.

• MUST-HAVE TESTS None

• OPTIONAL TESTS ApoE4 gene test for familial predisposition to Alzheimer's. COST: $200 TO $300

BLOOD AND HEART

• WHAT TO EXPECT For the hardest-working muscle in your body, the job doesn't get easier. Beginning in your forties and fifties, blood vessels tend to stiffen, causing the heart to work harder with each beat, which can raise blood pressure. And if plaque constricts the vessels, all this can set you up for a heart attack or stroke. (Fortunately, exercise helps ease the pressure on your heart. Think of it as cardiovascular insurance.)

• MUST-HAVE TESTS Blood pressure once every two years for men 18 and older. More frequently if your pressure exceeds 130/85. Starting at age 35, have your total cholesterol and high-density lipoprotein (HDL) cholesterol checked every five years. (High-risk men should start at 20.) COST: $12

• OPTIONAL TESTS C-reactive protein blood test (see page 120). COST: $60 Homocysteine blood test. COST: $50 The venerable Framingham Heart Study offers an online gauge of 10-year heart attack risk at hin.nhlbi.nih.gov/atpiii/calculator.asp.

PROSTATE

• WHAT TO EXPECT The doughnut-shaped prostate gland enlarges and gets denser as men march through middle age, increasing pressure on the bladder and leading to more frequent urination. That's merely bothersome. Worrisome is that one in six men will eventually get prostate cancer (about 70% of patients are over 65). New research shows that whether you are treated with surgery, external radiation or implanted radiation, the five-year survival rate is about the same: 99%.

• MUST-HAVE TESTS None

• OPTIONAL TESTS Digital rectal exam starting at age 50 as part of an annual checkup. COST: $10 TO $22 Prostate-specific antigen blood test. Start PSA screening at 50, or at 45 if you're African American or have a family history of the disease. COST: $26

SKIN

• WHAT TO EXPECT Forget the heart, lungs, even the beer belly: Your skin is the largest organ of the body. And between 40 and 50, its texture and appearance finally reflect the care (or lack thereof) you've given it since your teens and twenties. Old-school SPF sunscreens are a must. Newfangled skin-care lines aimed at men (the best have AHAs, Retinol and growth-factor proteins) also have the potential to slow time dermatologically. Need more motivation than that? Consider this: Men are more likely than women to develop malignant melanoma.

• MUST-HAVE TESTS None

• OPTIONAL TESTS Visual screening by a dermatologist or in your regular annual checkup. COST: ONE OFFICE VISIT

STOMACH AND COLON

• WHAT TO EXPECT By the end of your forties, there's a one in four chance you'll have at least one intestinal polyp. Most of these little protruding growths are harmless, but enough are malignant that 150,000 new cases of colorectal cancer are diagnosed each year. Having a parent or sibling with colon cancer worsens your odds from about one in 18 to one in 9. As with any cancer in the family, begin screening when you are 10 years younger than your relative was when diagnosed.

• MUST-HAVE TESTS Fecal occult blood test (to detect tumor- or polyp-related bleeding in stool) annually after 50 for men and women. COST: $27 Sigmoidoscopy (examining the lower colon) every five years. COST: $58 TO $125 Colonoscopy (examining all of the colon) every 10 years. COST: $210 TO $385

BODY FAT

• WHAT TO EXPECT After 30 your metabolism begins to slow and lean muscle mass is supplanted by fat. That can add about a pound a year in the absence of exercise. More than vanity is at stake. Obesity--often defined as being more than 30% over your ideal weight--is a key risk factor for America's top three killers (heart attack, cancer and stroke), as well as for Type II diabetes. A recent study has even linked it to greater risk of dementia and Alzheimer's. So, gentlemen, start your treadmills.

• MUST-HAVE DATA Body mass index (BMI), a ratio of weight to height. (You'll find a simplified calculator at nhlbisupport.com/bmi.)

NOTES: MONEY's "must have" tests have an A or B rating from the U.S. Preventive Services Task Force. "Optional" tests are widely recommended by experts. Cited costs are based on Medicare reimbursement rates or doctor estimates. The amount that private insurance and patients pay varies according to state regulations and many other factors.