Incubating cells at elevated temperatures causes an inhibition of protein synthesis. Mild heat stress at 41-42 degrees C inhibits the fraction of active, polysomal ribosomes from greater than 60% (preheating) to less than 30%. A return to 37 degrees C leads to an increase in protein synthesis, termed "recovery." Continuous incubation at 41-42 degrees C also leads to a gradual restoration of protein synthesis (greater than 70% of ribosomes reactivated by 2-4 h), termed "adaptation". Protein synthesis inhibition and reactivation is prestressed, recovered cells that contain elevated levels of the heat stress proteins occur to the same extent and at the same rate as in "naive" cells. The adaptation response requires transcription of new RNA whereas recovery does not. A large number of phosphorylation changes are induced by severe heat stress and occur with kinetics similar to the inhibition of protein synthesis. These include phosphorylation of eukaryotic protein synthesis initiation factor (eIF)- 2 alpha and dephosphorylation of eIF-4B and eIF-4Fp25 (eIF-4E). However, the extent to which the modification occurs is proportional to the severity of the stress, and, under mild (41-42 degrees C) heat stress conditions, these initiation factor phosphorylation changes do not occur. Similarly, under conditions of severe heat stress eIF-2 alpha and eIF-4B frequently recover to their prestress phosphorylation state before the recovery of protein synthesis. eIF-4E dephosphorylation likewise does not occur under mild heat stress conditions. Therefore, these changes in phosphorylation states, which are thought to be sufficient cause, are not necessary for the inhibition of protein synthesis observed.