Twenty-one day old rats were treated for 10 days with various doses of testosterone propionate (TP) (10 micrograms to 10 mg/day) and the levels of L-carnitine and testicular androgen binding protein (ABP) were measured in the 105,000 x g supernatant fractions of epididymis. Treatment with TP in increasing doses had a biphasic effect on the level of ABP in the epididymis; thus, with doses of TP of 10-100 micrograms/day, the ABP level was reduced in a dose-dependent way, whereas with higher doses of TP (0.2 to 1 mg/day) the extent of reduction of ABP levels was less as the dose of TP increased. Treatment with high doses (5 mg or 10 mg/day) did not change the ABP level compared with non-treated control rats. The concentration of carnitine increased linearly (log dose-response) with increasing doses of TP (10-200 micrograms/day) and there was no further increase after treatment with higher doses of TP. In adult rats TP (175 micrograms and 17.5 mg/day) reduced the level of ABP but not the level of carnitine in the epididymis. These studies suggest, therefore, that ABP is of minor importance for the supply of androgens to the carnitine-concentrating cells in the corpus and cauda epididymis.