Steady-state levels of the mRNAs encoding rat prostatic acid phosphatase (rPAP) were measured from the ventral prostate of rats after castration and testosterone replacement. The longest and the most abundant mRNA molecule (4.9 kb) of the rPAP gene was quite resistant to the hormonal status of the prostate, whereas its medium-sized transcript (2.3 kb) and the shortest one (1.5 kb) were up-regulated by androgens. For comparison, we also analyzed the prostatic mRNA levels of rat lysosomal acid phosphatase (rLAP), whose gene expression is not considered to be hormonally dependent. Only modest variation was observed in the rLAP transcript after androgen withdrawal and replacement. Corresponding protein level investigations are analogous with our regulation studies, particularly in the case of 1.5-kb rPAP mRNA.