Body weight gains of growing-finishing lambs fed practical diets were significantly (P<.05) affected by the addition of “soluble silica” (sodium silicate) to the drinking water at a concentration of about 800 mg per liter, expressed as SiO2. A significant (P<.05) interaction of “silica” × sex was noted, in which gains of males (wethers) were increased whereas gains of females were decreased by “silica,” indicating an effect at the tissue level. Feeding efficiency (feed per unit gain) was also affected by “silica” and the effect was moderated by nature of the diets which differed notably in roughage-concentrate ration and in urea vs. cottonseed meal as the main source of nitrogen.