A spontaneous, nonacute, age-dependent prostatitis was found in a high incidence in the lateral prostatic lobes of Lewis rats. Such rats were treated with methylprednisolone, indometacin , testosterone, hexyloxyphenylproprionate , polyestradiol phosphate, various antibiotics, or were caged together with female rats. The effect of the different treatment modalities was evaluated microscopically by blind observation of the degree of inflammatory reaction in the lateral prostate. Methylprednisolone and the testosterone ester caused a reduction of the inflammatory reactions and so did caging with female rats. The similarity of the rat prostatitis to the human condition may suggest the possibility of using corticosteroids or androgens for the treatment of patients with nonacute prostatitis.