Somatolactin (SL) is a fish pituitary hormone belonging to the growth hormone/prolactin family. Since its discovery in 1990, a number of cellular and molecular studies have been performed to unravel the structure and function of this protein. Amino acid and nucleotide alignments of the SL-coding regions indicate that the SL gene is highly conserved. The 26 kDa protein was first isolated from pituitary glands of Atlantic cod, and it has not been yet identified in higher vertebrates. The expression of the SL gene, as well as those of growth hormone (GH) and prolactin (PRL). is regulated by a pituitary-specific transcription factor named Pit-1. The SL-producing cells were identified in the pars intermedia of pituitary gland of most fish species as periodic-acid-Schiff positive cells. Recent observations confirmed that the pituitary is the predominant tissue in SL production but, most probably, it is not the only one in which SL gene is expressed. The target tissue for the biological activity of SL has not been yet conclusively identified. However, it has been suggested that this pituitary hormone plays a significant role in gonadal maturation and spawning, hydromineral and acid-base regulation, background adaptation, and energy mobilization. Studies on the evolution of pituitary hormones and the target organs and receptors would spot some light on the physiological significance of SL and their relationship with GH and PRL.