Between 24 and 40 hr after cadmium treatment in the rat, whilst the testis and caput epididymidis are undergoing haemorrhagic necrosis, the cauda epididymidis and contained spermatozoa remain morphologically normal. Sexual activity is diminished but fertile matings occur. By 7 days, reduced androgen output results in atrophic changes in the accessory glands, cauda epididymidis and vas deferens, in which spermatozoa are now degenerate. Few males mate and these are infertile. The changes in the cauda can be prevented by testosterone treatment and fertility remains normal up to 9 days after the dose of cadmium.