The effects of intratesticular injection of naloxone, a universal opioid antagonist, on testicular immunoreactive (IR)-arginine vasopressin (AVP) content and on in vitro testosterone production by Leydig cells were investigated in the mouse. Bilateral intratesticular injection of increasing doses of naloxone (0.1-10 micrograms/testis) resulted 24 h later in a dose-dependent increase in testosterone production by Leydig cells incubated for 3 h in the presence or absence of hCG (100 ng/ml). Unilateral intratesticular injection of naloxone (10 micrograms) similarly enhanced basal and hCG-stimulated testosterone production by Leydig cells, but production was not modified in Leydig cells from the contralateral vehicle-injected testis, nor was it changed when the same dose was injected subcutaneously. Unilateral intratesticular injection of 10 micrograms naloxone led to a dose-dependent increase in the hCG-responsiveness without altering the slope of the hCG dose-response curve. In vitro exposure of Leydig cells to increasing doses of naloxone (10(-9) to 10(-7) M) did not alter either basal or hCG-stimulated testosterone production. Testicular IR-AVP content declined in a dose-dependent manner in naloxone-injected testis, but remained unchanged in the contralateral vehicle-injected testis and in testis from animals that received similar doses of naloxone subcutaneously. Since AVP has been shown to locally exert a negative control on testosterone production within the testis, it might be hypothesized that the increased Leydig cell activity after local naloxone administration results from reduced intratesticular IR-AVP levels.(ABSTRACT TRUNCATED AT 250 WORDS)