The experiments reported show that one of the early effects of the steroid sex hormone antheridiol is on the synthesis of rRNA and ribosomes. This is demonstrated in hormone treated cultures of Achlya ambisexualis (strain E87) by an enhancement of the incorporation of [3H]uridine into 26S and 18S rRNA and by an increase in measurable amounts of ribosomes per milligram dry weight of mycelium. Furthermore, since the hormone does not significantly alter the pool size or the specific activity of uridine triphosphate, this effect appears to represent an increased rate of RNA synthesis.