The ability of ovine growth hormone (oGH) to enhance the hypo-osmoregulatory performance of a non-migratory stock of the brown trout (Salmo trutta) was investigated. Three groups of juvenile fish were intraperitoneally implanted either with a cholesterol pellet (sham) or with a cholesterol pellet containing 250 μg oGH (treated); control fish did not receive any implant. While still in fresh water, the gill Na⁺⁄K⁺-ATPase activity of the oGH-treated group was four times higher than that of sham and control groups 2 weeks after implantation. Transfer to sea water (SW) resulted in marked increases in plasma electrolyte levels of the sham and control groups, whereas the oGH-treated group showed only minor perturbations in plasma electrolyte concentrations. Subsequent regulation of plasma electrolyte parameters to lower levels was slower or less effective in the non-treated groups than in the oGH-treated group. In the non-treated groups, SW exposure brought about significant increases in gill Na⁺⁄K⁺-ATPase activity but only after a time lag of 7 days. An additional significant increase in gill Na⁺⁄K⁺-ATPase activity was also observed in the oGH group 7 days after SW exposure. By the end of the experiment, oGH-treated fish were significantly larger than non-treated ones. The data indicate that after oGH administration, juvenile non-smoltifying brown trout respond to SW exposure in the same manner as a fully smolted salmonid.