Insulin-like growth factor (IGF)-I, which is crucially involved in fish growth, differentiation, and reproduction, occurs in tilapia pituitary. IGF-I peptide, which is probably produced in hypothalamic perikarya, is present in axons of the neurohypophysis, and IGF-I mRNA and peptide are present in the adenohypophysis in adrenocorticotrophic hormone cells,melanocyte-stimulating hormone cells, with interindividual differences in growth hormone cells, and, starting with puberty, in gonadotrophic hormone (GTH) cells. Subordinate males showed a high IGF-I but a lower ^-luteinizing hormone expression, while in dominant males the opposite was found. IGF-I from the GTH cells may act as auto/paracrine regulators of GTH cell proliferation and enhance GTH synthesis and release during puberty and reproduction, depending on the social status.