Studies were undertaken to determine whether several indicators of growth hormone (GH) cell activity, namely GH content, fine structure, and volume of the GH region, differ in the pituitaries of freshwater (FW) and seawater (SW) tilapia, Oreochromis mossambicus. Tilapia raised from the stage of yolk-sac absorption for 7 months in SW contain significantly more GH in their pituitaries than in those of fish reared in FW. Pituitary growth hormone content in tilapia raised in FW for 7 months and transferred to SW for 49 days is greater than that in sibling tilapia retained in FW. Conversely, GH content is significantly lower in the pituitaries of SW-reared tilapia transferred to FW for 49 days than that in the pituitaries from fish retained in SW. Likewise, the volume of the GH region and activity of the GH cells are enhanced in pituitaries from SW-reared tilapia over that seen in pituitaries from FW fish. Taken together, all data indicate heightened GH cell activity in SW-raised tilapia and suggest that GH may play a causal role in the greater growth rates observed in SW tilapia compared to FW fish and/or that GH may be involved in SW osmoregulation. The latter suggestion is supported, in part, by our observation that in vivo oGH treatment (2 micrograms/g body wt) stimulated gill Na+,K(+)-ATPase activity.