The first of a series of experiments designed to study the effects, if any, of cadmium on the prostate gland is reported. The repeated subcutaneous injection of cadmium sulphate into the flanks of rats gave rise to a low incidence of sarcomata at the injection sites. At the level of 0.2mg 3CdSO4.8H2O (0.087 mg Cd) once weekly per animal for 2 yr there was a significant inhibition of weight gain. This effect was not seen at the levels of 0.1 or 0.05mg 3CdSO4.8H2O once weekly. No neoplastic changes were seen in the prostate gland at any level of exposure. At all levels of exposure, cadmium accumulated in the spleen, liver, kidney and testis. Accumulation was most marked in the kidney. Exposure to cadmium in the concentrations used had no observed effect, either qualitative or quantitative, on the proteinuria which is characteristic of laboratory rats as they get older. A high incidence of testicular changes and of Leydig-cell tumours was common to cadmium treated and control rats. Subsequent experience has indicated that this is a feature of the subline of C.B. hooded rats used for the experiment. A low incidence of neoplasms of other sites was seen in all four groups and was not associated with treatment.