We have examined the mode of morphine's (M) action to increase the sensitivity of castrated male rats to the inhibitory feedback action of testosterone (T) on LH release. In castrated rats, sc implantation of M pellets or 5-mm long T-filled capsules (T5) failed to suppress LH release, but a combination of M and T5 drastically decreased serum LH levels. Likewise, while treatment with a higher dose of T (30-mm long implant, sc) suppressed LH release, combined treatment with M and T30 produced a further suppression of LH levels. We have now assessed the in vitro release rate of LHRH from the medial basal hypothalamus-preoptic area of castrated rats treated with M and/or T as well as the in vivo pituitary LH response to LHRH challenge in similarly treated rats. Interestingly, the in vitro basal and naloxone-induced LHRH release from the medial basal hypothalamus-preoptic area of the six groups of rats was similar, regardless of whether LH levels were in the high castrate or low basal range. On the other hand, M treatment greatly attenuated LH release in vivo in response to LHRH challenge (10pmol-1pmol) [corrected] in T-treated rats. In fact, LH increments in response to 1pmol [corrected] LHRH, seen in control, T5, and T30 groups, were abolished by additional M treatment of T-treated rats. This in vitro assessment of LHRH release suggests that the drastic decrease in LH release in T-plus M-treated rats may not be due to impaired LHRH release, but, rather, be due in part to reduced pituitary responsiveness to intermittent endogenous LHRH signals. The reduced pituitary responsiveness to LHRH in T-plus M-treated rats may be a consequence of either a direct pituitary effect of opiates in conjunction with T or augmented action of hypothalamic neurohumoral agents which may inhibit LH release on their own or antagonize the LH-releasing action of LHRH at the level of pituitary gonadotrophs.