Compared with female rats a distinctly higher rate of accumulation of p-aminohippurate (PAH) in renal cortical slices from mature male rats can be proved; there are no sex differences in young rats with immature kidney function. Postnatal development of sex differences in kidney weight shows the same pattern. The lack of sex differences in the renal excretion of PAH in all age groups could be caused by a higher fraction of glomerularly filtered PAH in female rats. In mature male rats the high rate of tubular transport of PAH can be diminished by castration or by blockade of testosterone receptor sites (treatment with cyproterone). Stimulation of PAH transport by repeated treatment with testosterone is more expressed in male than in female rats. Furthermore, estrogens seem not to be responsible for sex differences in renal tubular transport of PAH (no influence of ovarectomy, low effects of treatment with estradiol in male rats).