The effects of subcutaneous and medial basal hypothalamic (MBH) implantation of testosterone (T) on LHRH contents and norepinephrine (NE) and dopamine (DA) concentrations and turnover in the MBH and the preoptic area (POA) were studied in castrate rats; serum levels of LH, FSH and T were also monitored. Subcutaneous T implants elevated serum T levels to those normally observed in intact rats and this resulted (i) in marked increments in the MBH LHRH content and reduced serum LH and FSH levels, (ii) in reduced turnover of NE without altering that of DA in the MBH, and (iii) in a concomitant decrease in the POA DA turnover rate. Following implantation of T in the MBH there was no significant rise in serum T concentration. As in the case of s.c. T implants, these MBH implants suppressed serum gonadotropin levels and reduced the turnover of the MBH NE without affecting that of DA. The effects of the MBH T implants were apparently local since the turnover of neither catecholamine in the POA was altered. From these observations it is reasonable to conclude that (i) hypothalamic NE innervations possess the ability to respond to changes in T titers, (ii) the effects of T on NE activity are exerted within the MBH and accordingly, (iii) these changes in the MBH NE turnover following T administration may play a role in regulation of the MBH LHRH activity.