NEW YORK (CNN/Money) - Sirius shares lost almost a quarter of their value Wednesday after an investment bank said the company was probably overvalued.

Sirius (Research) tumbled 23.5 percent to close at $6.90 after a Smith Barney analyst said the satellite radio company had risen to a valuation that was difficult to justify. Smith Barney then downgraded the company's shares to "sell" from "hold."

Sirius stock had almost tripled in the last two months after attracting attention by adding radio industry veteran Mel Karmazin as its new chief executive and revealing plans to broadcast Howard Stern's popular radio show in 2006.