A study was carried out in tilapia in order to see whether the gonadotropin (GtH) β subunits show distinct patterns of expression at different stages of their reproductive development. Male and female tilapia hybrids (Oreochromis niloticus × O. aureus) were collected at various times of the year, and a number of parameters were measured in order to establish the reproductive state of the fish. Circulating testosterone (T), estradiol (E 2 ) and 11 ketotestosterone (11KT) levels were assayed, gonads were removed for calculation of gonadosomatic index (GSI) values and histological studies, and RNA was extracted from the pituitaries for measurement of GtH Iβ and IIβ mRNA levels. In maturing fish of both sexes, the circulating steroid levels were positively correlated with each other (r 2 = 0.66-0.91) and in males, also with the GSI values (r 2 = 0.68). A positive correlation was also seen in these fish between GSI values and the prevalence of spermatocytes and spermatids (r 2 = 0.54). In maturing females, the maximal oocyte diameter was positively correlated with circulating E 2 levels (r 2 = 0.63), while GSI values showed no correlation; this presumably relates to the cycling nature of this asynchronous spawner. In regressing fish of both sexes, no clear correlation between these reproductive parameters was seen. In all fish, the GtH Iβ mRNA levels were highest in fish with steroids ranging 1-10 ng T or E 2 /ml for males or females, respectively, and were lower in fish with steroids at higher or lower levels. In fish with high steroid levels, the IIβ mRNA levels were also high, and in regressed males the increases were positively correlated. Exposure of cultured pituitary cells to either steroid (T at >10 nM, or E 2 at >1 nM) was followed by a decrease in the steady-state levels of the Iβ transcript, while those of IIβ were left unaltered. In situ hybridization studies revealed that in pituitaries of both sexes, the cells producing each of these mRNAs are located in a distinct location. These results suggest that gonadal steroids may exert differential feedback mechanisms at the level of the pituitary to control transcription of each GtH β subunit in distinct cell types specific for each hormone.