Heat shock induced by an increase in temperature from 28 to 37 degrees C led to changes in synthesis and phosphorylation of cytoplasmic and nuclear proteins in the aquatic fungus Achlya. In the cytoplasmic fraction a marked increase in [35S]methionine labelling of proteins in the molecular weight range of 96 000, 85 000, 74 000, and 70 000 was observed. Two-dimensional electrophoresis resolved each of these classes of proteins into several components. Major changes in the nuclear fraction included the increased [35S]methionine labelling of 43 000 and 28 000-23 000 proteins. A marked decline in the synthesis of many other proteins was also evident. The heat-shock-induced changes in labelling patterns became evident as early as 20 to 60 min after treatment, but they were transient. With continued incubation at the heat-shock temperature, the cells appeared to adapt to the new temperature conditions. Both cytoplasmic and nuclear proteins returned to nearly normal labelling patterns within 100 to 140 min at 37 degrees C. Changes in phosphorylation of histone and nonhistone nuclear proteins were also noted. Achlya histone H3 and the putative oomycete-specific histone "alpha" appeared highly phosphorylated after heat shock. Since phosphorylation of histone H3 is primarily associated with chromatin condensation, it is possible that rapid chromatin condensation is an initial response to heat shock in Achlya.