Assessments of the reproductive toxicity of organophosphorus insecticides are important public health issues. This study aimed at defining the testicular toxicity of dichlorvos (DDVP) since this toxicity was suspected by our previous survey on pesticide sprayers and in some earlier publications during the 1970s. Ten-week-old Wistar rats were divided into four groups (n=8 or 9) and were injected subcutaneously with DDVP (0, 1, 2 or 4 mg/kg) 6 days a week for 9 weeks. After that period, erythrocyte cholinesterase (ChE) activities decreased dose-dependently, showing 44-55% inhibition among the treated groups. No significant difference was observed in the reproductive organ weights in any treated groups compared with the control group. Sperm motility decreased slightly but significantly in the 1 and 4 mg/kg groups, and significant regressions were observed between sperm motility and both blood ChE activity and urinary concentration of dimethyl phosphate (DMP), a urine metabolite of DDVP. However, sperm counts and sperm morphology in the cauda epididymidis, plasma testosterone concentrations, and histopathology in the testes in all the treated groups were not significantly different from those of the control group. Since only the sperm motility deteriorated by DDVP exposure at doses inducing marked inhibition of cholinesterase activities in the rats, it was suggested that the risk of testicular dysfunction posed to occupationally exposed humans would be small in terms of the effect of DDVP exposure alone. This conclusion was also supported by an estimate of the decrease in human sperm motility based on the urinary DMP concentrations observed in actual occupational settings.