Tetrahymena were grown in proteose-peptone medium supplemented with glucose, mannose, fructose, galactose, acetate, succinate, or pyruvate and then washed and resuspended in a non-nutrient salt solution and the amounts of 7 acid hydrolases secreted into the medium in a one hour incubation were measured. Cells that had been grown in the presence of glucose secreted about half the amounts of acid phosphatase, beta-N-acetylglucosaminidase and acid protease as did control cells grown in unsupplemented medium. Pyruvate was about as effective as glucose and both were slightly more effective than acetate or fructose. Succinate had little effect. Similar experiments showed that alpha-mannosidase, beta-fucosidase, and beta-galactosidase are secreted into the salt solution and the secretion is reduced by prior growth of the cells in medium supplemented with glucose or mannose but not galactose. Except for alpha-mannosidase, these reductions in amounts of hydrolase secreted were not accompanied by appreciable changes in intracellular activity, and therefore demonstrate a persistent effect of growth in the presence of certain metabolites on the subsequent secretion of lysosomal hydrolases. Since the inhibition of subsequent secretion depended on both the individual metabolite and the particular hydrolase examined, it appears that the effect of metabolites is not limited to a general inhibition of secretion but may differentially alter some properties of lysosomal subpopulations. A preliminary characterization of the secreted acid protease of Tetrahymena suggests that there may be two acid proteases released, since up to 25% of the activity was not inhibited by high concentrations of pepstatin, leupeptin, or chymostatin.