When exponentially growing cultures of Achlya ambisexualis strain E87 were raised from their normal growth temperature of 30 degrees C to 35 degrees C, the rates of synthesis of a small number of proteins were dramatically increased. The most predominant proteins synthesized in response to heat shock had molecular weights of 70,000 and 78,000, and their increased synthesis was detected as early as 10 min after the shift to 35 degrees C. Changes in the populations of translatable messenger RNAs during heat shock showed that the levels of the mRNA's for all the major induced proteins correlated very closely with the alterations in the in vivo patterns, suggesting a transcriptional level of control of their synthesis. When after a period of heat shock (60 min) the cultures were shifted back to 30 degrees C, recovery of the preshock patterns of protein synthesis was attained after several hours. Different proteins show temporally distinct patterns of recovery. During recovery the levels of translatable mRNA's for the induced proteins also correlated closely with the patterns of in vivo protein synthesis.