The levels of androgens in the male prostate, seminal vesicles, and serum, as well as in the female prostate and serum, were measured in the rodent Praomys (Mastomys) natalensis. Measurements were made in different age groups of both males and females using a sensitive radioimmunoassay. The results showed that the values for serum testosterone were 10 times greater in the male than the female. The concentration of serum dihydrotestosterone in the male was approximately one-tenth of the concentration of testosterone. The recovery of dihydrotestosterone from the prostate in the female and seminal vesicle in the male was low and these values were not considered. The concentrations of testosterone and dihydrotestosterone in the male prostate were 8.3 +/- 0.5 and 20.0 +/- 2.4 ng per g of wet tissue, respectively. The concentration of testosterone in the female prostate, 7.1 +/- 0.5 ng per g of wet tissue, was similar to that in the male. These studies have shown that the female Mastomys has a prostate which exists in a low androgen environment; it is suggested that this could be used as an experimental model for studying hormonal effects on the prostate.