&NA;The effects of halothane on uptake and phosphorylation of uridine, and on cellular ATP content were studied inTetrahymena pyriformis,a ciliate protozoan. Exposure to halothane inhibited the accumulation of14C‐uridine into the following acid‐soluble intracellular pools: UTP, UDP, UMP, and an unidentified compound. Halothane did not alter ATP content of intact cells. It is concluded that inhibition by halothane of uridine incorporation into RNA ofT. pyriformisis due to effects on uridine uptake and/or phosphorylation and not due to inhibition of the RNA polymerase reaction or reduction of ATP content.