The effect of ethanol and heat on the thermosensitivity of 3-day-old larvae of the fresh water snail Lymnaea stagnalis is investigated, especially with regard to the kinetics of thermotolerance, the effect on protein synthesis and the pattern of proteins synthesized. Both stress factors, in a mild dose, induce a state of thermotolerance with the following characteristics: (i) it is not accompanied by an enhanced synthesis of HSPs, (ii) it needs only 10-30 min to develop maximally, and (iii) it decays within 60-90 min after it has been triggered. At a higher dose both factors induce an enhanced synthesis of the HSP 65, HSP 70, and HSP 87, and also a more stable state of thermotolerance beside the thermotolerant state present shortly after the trigger. It appears that the synthesis of HSPs is enhanced only when the overall protein synthesis is depressed. The data are discussed in relation to the putative functions of heat shock proteins. It is suggested that the constitutive levels of the HSPs provide the rapid protection against heat. An induced level of HSPs is necessary for the extension of the thermotolerant state.