The effects of heat on mouse spermatogenesis have been determined using both testis weight and flow cytometrically determined DNA content distribution as experimental end-points. Temperatures of 38-42 degrees C and exposure times of 20-60 min have been tested. The results concerning the testis weight substantially confirm those reported by other authors (Hand et al. 1979). The measurement of DNA content distributions shows a relatively higher depletion, 14 days after treatment, of the cytometric compartment containing elongated spermatids in respect to that containing round spermatids. The analysis of the cytotoxic effects, monitored 14 vs. 28 days after treatment, as a function of the exposure time at a given temperature, or of the temperature for a fixed exposure time, indicates that, in the course of spermatogenesis, late spermatocytes are more sensitive to heat than differentiated spermatogonia. Following the approach based on flow cytometry, the effect of exposures as low as 20 min at 38 degrees C can be appreciated.