Using in situ hybridization, messenger RNAs for gonadotropin-releasing hormone (GnRH) were seen distributed differently in the brains of 40-day-old juvenile tilapia Oreochromis mossambicus. While salmon-GnRH mRNA was localized in the nucleus olfactoretinalis (terminal nerve ganglia), chicken II-GnRH mRNA mRNA was observed in the midbrain nucleus. Various concentrations (0.1-10 mg) of estradiol benzoate and triiodothyronine, given over a 24 h period, had no effects on mRNA levels of salmon- and chicken II-GnRH. Analysis of variance indicated significantly higher levels of salmon- but not chicken II-GnRH mRNA in larger (> 1.5 mm) versus smaller (1.3 mm) brains, among juveniles of the same age and same genetic brood. Thus, salmon-GnRH neurons in the nucleus olfactoretinalis display greater variance depending on the body mass. Since reproductively active tilapia differ with respect to body size at sexual maturation, therefore, besides the age and treatment effects, body size should be taken into consideration.