The phenomenon of thermotolerance has been studied in three-day-old larvae of Lymnaea stagnalis (Pulmonata, Mollusca). The lethality of a test treatment (TT) of 1 h, 40.0 degrees C is greatly reduced by pretreatment at elevated temperatures. The induced thermotolerance shows characteristics which are dependent on duration and temperature of the pretreatment (conditioning treatment; CT). Thermotolerance induced by a CT of 1 h at greater than 38.0 degrees C is stable at least during 2 h at 25 degrees C, whereas thermotolerance induced by the lower temperatures decays partly. Five minute CTs at 33 and 35 degrees C induce thermotolerance as well; however, this fades completely within 1 h at 25 degrees C. The same level of tolerance is present after a 5 min CT at 38 degrees C, but this level continues to rise during 1 h at 25 degrees C. The induction of thermotolerance (CT 1 h) is suppressed only partly when protein synthesis is inhibited by puromycin. The effect of puromycin is relatively weaker at the lower conditioning temperatures. Furthermore, puromycin slows down the decay of thermotolerance. The influence of duration and temperature of heat treatment on the pattern of protein synthesis has been investigated by gel electrophoresis after incubation in [35S]methionine. Heat-shock proteins (hsp) are synthesized during 1 h incubation at 35, 37, and 38 degrees C, but not at 33 degrees C. Twenty-four hours at 33 degrees C does induce the synthesis of hsp. It is proposed that thermotolerance in Lymnaea is based on two states, a rather unstable one which is independent of hsp synthesis, and another more stable one, accompanied by hsp synthesis.