The gonadotropin (GtH) II mRNA levels in pituitary glands of male tilapia hybrids were proportional to the gonadosomatic index (GSI), while GtH I was highest in fish with GSI of 0.56, but was lower in fish with more developed testes. In pituitary cells of immature fish, only GtH I mRNA responded to testosterone (T) exposure (12–72h), rising nearly three-fold compared to the control cells. The stimulatory effect of T on GtH I in these fish was also seen to be dose-related, but doses of 100 nM and above were ineffective. In pituitary cells of mature fish, both transcript levels rose on exposure to 0.01–0.1 nM T, but at higher concentrations, the GtH I fell to control levels, while those of GtH II continued to increase. Growth hormone (GH) mRNA levels were not affected by any of the T treatments. These results suggest that T may differentially control expression of the two GtH subunits; low levels in immature fish stimulating the I but not the II mRNA, while higher levels, in maturing fish, stimulate the II but not the I transcript.