Methyl testosterone (MT) and 17 beta-estradiol (E2), suspended either in sesame oil (and injected on alternate days) or in warm hydrogenated coconut oil (and injected once in the form of an "implant"), were administered intraperitoneally to nonvitellogenic rainbow trout (Salmo gairdneri Richardson). When given as sesame oil injections E2 significantly reduced plasma triiodo-L-thyronine (T3) levels and thyroid epithelial cell height (TEH), but had no apparent effect on plasma L-thyroxine (T4) levels. When given as a coconut oil "implant" E2 significantly reduced both plasma T3 and T4 levels. E2 also elevated the hepatosomatic index (HSI), plasma Ca2+, Mg2+, protein, triglyceride, and alkali-labile phosphoprotein (ALP) and hepatic lipid concentration, decreased hepatic glycogen levels, but was without effect on plasma Na+ and glucose levels. MT given via sesame oil injections significantly lowered plasma T3 levels, but was without effect on plasma T4 concentrations or TEH; the T3-lowering effect of MT was not evident when the steroid was given in the form of a coconut oil "implant." MT also effected a lowering of plasma Mg2+ and protein and hepatic lipid concentrations, but had no significant effect on HSI, plasma Na+, glucose triglyceride and ALP concentration, or hepatic glycogen levels. MT given as a series of injections lowered plasma Ca2+ levels, but was without effect when given as an "implant" and had no significant effect on plasma Ca2+. Intraperitoneal injections of ovine thyrotropin (TSH) (2 micrograms/g body wt) elicited an increase in both plasma T4 and T3 levels within 24 hr in E2-, MT-, and oil-injected groups. The TSH-stimulated increase in plasma T4 level was significantly less in the E2-treated fish compared with oil- or MT-treated groups. The TSH-stimulated increase in plasma T3 levels in both groups of steroid-treated fish was significantly less than that of the oil-injected controls.