Trachea from albino mice were cut transversely into nearly identical rings and incubated in medium 199 with Hanks salts and HEPES buffer at 37°C. Sodium selenite at 0.5–5 mMdepressed the ciliary activity. With 1 and 5 mMsodium selenite, a 50% reduction in the activity index was observed after approximately 5 and 1.5 h, respectively. The ATP content in trachea rings was reduced with 0.05–5 mMsodium selenite, and increasing concentrations gave decreasing amount of ATP after incubation for 4 and 21 h. The rate of protein synthesis as determined by incorporation of radioactive leucine was reduced with 0.5 and 2 mMsodium selenite. The synthesis was reduced quickly by 2 mMsodium selenite, which gave a 30% reduction after incubation for 1 h.