The influence of season on testicular steroid production as a parameter of testicular function has been studied in a wild boar. Semen was collected once weekly while it served the dummy. In seminal plasma concentrations of the following steroids were determined by radioimmunoassay: unconjugated testosterone, conjugated testosterone, unconjugated total oestrogens, conjugated total oestrogens and 5 alpha-androst-16-en-3-one ('boar-taint steroid'). All steroids showed a clear seasonal pattern with highest concentrations in autumn and early winter and low levels from January to July. Maxima during the rutting season were 10-25 times greater than average values out of season. During a 2-month-period (mid-July until mid-September) libido was abolished and the wild boar refused to mount the dummy. These results indicate that the seasonal variation in testicular steroid production by the wild boar, regulated by photoperiod, are similar to those of the domestic boar.