The time course and magnitude of the heat-shock response in relation to severity of thermal stress are important, yet poorly understood, aspects of thermotolerance. We examined patterns of protein synthesis in congeneric marine snails (genus Tegula) that occur at different heights along the subtidal to intertidal gradient after a thermal exposure (30 degrees C for 2.5 h, followed by 50 h recovery at 13 degrees C) that induced the heat-shock response. We monitored the kinetics and magnitudes of protein synthesis by quantifying incorporation of 35S-labeled methionine and cysteine into newly synthesized proteins and observed synthesis of putative heat-shock proteins (hsp's) of size classes 90, 77, 70, and 38 kDa. In the low- to mid-intertidal species, Tegula funebralis, whose body temperature frequently exceeds 30 degrees C during emersion, synthesis of hsp's commenced immediately after heat stress, reached maximal levels 1-3 h into recovery, and returned to prestress levels by 6 h, except for hsp90 (14 h). In contrast, in the low-intertidal to subtidal species, Tegula brunnea, for which 2.5 h at 30 degrees C represents a near lethal heat stress, synthesis of hsp's commenced 2-14 h after heat stress; reached maximal levels after 15-30 h, which exceeded magnitudes of synthesis in T. funebralis; and returned to prestress levels in the case of hsp90 (50 h) and hsp77 (30 h) but not in the case of hsp70 and hsp38. Exposures to 30 degrees C under aerial (emersion) and aquatic (immersion) conditions resulted in differences in hsp synthesis in T. brunnea but not in T. funebralis. The different time courses and magnitudes of hsp synthesis in these congeners suggest that the vertical limits of their distributions may be set in part by thermal stress.