After i.m. injection of [3H]butyrobetaine into rats, the accumulation of carnitine into the epididymis, prostate gland, seminal vesicles, testis and heart was studied. The concentration of radiolabeled carnitine into the cauda epididymis increased linearly with time up to 72 h after the injection of the precursor, while its level in the prostate and seminal vesicles decreased rapidly. Very low levels of carnitine were found in the testis. Castration reduced the carnitine accumulation by cauda epididymis to 6% of the control levels while treatment of castrated animals with testosterone propionate (500 mug/day) partly restored the carnitine uptake. Similar treatment with 17beta-oestradiol valerate or 17alpha-hydroxyprogesterone had no effect. Surprisingly, cyproterone acetate (5 mg/day) also significantly stimulated carnitine accumulation by the epididymis to a level above that of the castrated controls. Simultaneous injection of both cyproterone acetate and testosterone propionate to castrated animals caused an additive effect of these steroids. This indicated that cyproterone acetate in this system is working as a weak androgen. Treatment of rats with 17beta-oestradiol valerate also reduced carnitine accumulation by the cauda epididymis. This is due to suppression of pituiatry gonadotrophin secretion, since concommitant treatment with testosterone propionate (500 mug/day) caused a normalization of the carnitine uptake. Treatment of intact rats with cyproterone acetate significantly reduced the epididymal weight, but not the carnitine accumulation. 17alpha-Hydroxyprogesterone treatment had no effect either on the epididymal weight or the accumulation of the carnitine. Unilateral orchiectomy reduced the carnitine accumulation by the cauda epididymis to about 40% of that occurring in the non-operated control side. This indicates that the luminal contact between the testis and epididymis or the luminal content of the epididymis itself is of importance for the androgen-dependent metabolic process occurring in the cauda epididymis. Castration or hormone treatment did not change the conversion of butyrobetaine to carnitine, or the carnitine uptake by heart. Carnitine uptake by the testis after [3H]butyrobetaine injection was rather low and this would exclude the possibility of synthesis of carnitine in the testis as a source of epididymal carnitine. Carnitine only accumulated in the cauda epididymis in vivo 4 to 96 h after injection of [3H]butyrobetaine. The presence of radioactively labeled butyrobetaine or methylcholine was not detected.