Pituitary gonadotrophs were studied in male African catfish between 1 and 37 wk of age using antisera against the LH subunits for immunohistological and radioimmunological purposes, and cRNA probes for in situ hybridization. Immunoreactive material was already detectable at the earliest age examined. In juveniles, the signal for the common glycoprotein alpha subunit (GP alpha) was stronger than that for the LH beta subunit. Accordingly, an excess of radioimmunoassayable GP alpha 100 times that of LHbeta was recorded in the pituitary. Using in situ hybridization, the mRNAs were detected 7 (GP alpha) and 13 (LHbeta) wk after hatching. Detection of LHbeta mRNA coincided with a 300-fold increase in the pituitary content of LHbeta and intact LH, whereas GP alpha increased only 15-fold. The number of gonadotrophs per pituitary and the amount of LH per gonadotroph also increased strongly. The strong, initial increase in pituitary LH levels was always associated with the presence of spermatocytes. However, in a limited number of cases (3 out of 12 fish), the pituitary LH content was low despite the presence of spermatocytes. The number of gonadotrophs, the staining intensities (reflecting protein and mRNA), and the pituitary LH content kept increasing, although at a reduced rate, until completion of the first wave of spermatogenesis. In view of the excess of GP alpha over LHbeta, we conclude that expression of the two subunits is regulated in part by different mechanisms, and that expression of LHbeta is rate-limiting for the amount of intact LH. The strong activation of the gonadotrophs shortly after meiosis opens the possibility that a signal of testicular origin stimulates LH expression, in particular its beta subunit. In the absence of a FSH-like gonadotropin in catfish, we propose that LH covers all functions requiring gonadotropic regulation in the African catfish.