Tolerance of both protein synthesis and seedling growth to a previously lethal high temperature can be induced by prior exposure to a sub-lethal temperature during which the synthesis of heat shock proteins (HSPs) occurs. In this study, a thermal gradient bar was used to measure the physiological effects of temperature on seedlings of sorghum (Sorghum bicolor L.) in conjunction with studies of gene expression. The duration of HSP synthesis, both during continued high temperature treatment or on return to normal temperatures, was found to be very finely modulated and was dependent on the severity of the initial heat shock. The synthesis of heat shock proteins and the induction of thermotolerance were rapid, reversible and reinducible phenomena. Maximal thermotolerance was obtained after treatments that induced the full complement of HSPs. Subsequent treatments that repressed HSP synthesis, also abolished thermotolerance. The presence of HSPs, however, was not sufficient for the tissue to be in a thermotolerant state and the results suggest that either their de novo synthesis, or some other factor, is required for the induction of thermotolerance. Pre-existing HSPs did not inhibit the synthesis of new HSPs. Although the kinetics of the synthesis of HSPs and the development of thermotolerance show a tight correlation, the kinetics of the decay of thermotolerance and the degradation of HSPs were not linked. The functional state or distribution of HSPs may well change during the recovery process.