Cordycepin (3′-deoxyadenosine), at a dose of 10 mg/kg, inhibits the accumulation of ribosomes in rat liver cytoplasm within 2.5 h of administration. This effect is not due to an inhibition of RNA synthesis per se but rather to the premature termination of the transcription of the 45 S ribosomal precursor in the presence of the analogue. The nucleoside analogue also inhibits the corticosteroid-mediated induction of tyrosine transaminase and general protein synthesis. The latter results are consistent with the current theory that cordycepin inhibits the formation of polyadenylic acid which is involved in the nuclear processing and/or nucleocytoplasmic transport of messenger RNA.