Preheating of Neurospora conidia increased their susceptibility to mutation induction by chemical mutagens. Optimal conditions of heat shock for enhanced mutagenesis were determined in 2.5 X 10(7) conidia/ml 0.067 M KH2PO4-Na2HPO4 (pH 7.0) buffer to be treatment at 43 degrees C for 60 min. When protein synthesis during heat stock was eliminated by cycloheximide or by use of the temperature-sensitive mutation psi-1, induction of thermotolerance was inhibited while induction of the enhanced state of mutability was not. Therefore, inducible protein synthesis is not involved in this process. To discover whether DNA-repair systems are altered by heat shock and, as a result, whether reversion frequencies increase, DNA-repair mutants (upr-1, uvs-2, uvs-3, uvs-6, mus-7, mus-16) were heated and their reversion frequencies at the ad-8 locus were measured. All the DNA-repair mutants showed higher reversion frequencies with MNNG treatment after heat shock than in non-heated control. It therefore seems that DNA repair is not involved in the enhancement of chemical mutagenesis by heat shock. Heat shock does not increase frequencies of reversion induced by ultraviolet light, and heat shock after treatment with chemical mutagens does not affect reversion frequencies. These results suggest that heat shock may change the structure and function of cellular membranes and thereby increase the influx of mutagens into cells.