Abstract Insulin or one of the following compounds—l-cysteine, reduced glutathione, 2-mercaptoethanol, or dithiothreitol—was incubated with isolated fat cells or broken fat cells from rats. In a concentration range of 0.01 to 1 mm, these effects of thiols resembled those of insulin: (a) partial suppression of lipolysis induced by epinephrine, the degree of inhibition being inversely related to the concentration of epinephrine, (b) increase of glucose utilization, with preferential utilization of glucose carbon for fatty acid synthesis and the pentose cycle, and (c) stimulation of glucose metabolism in intact, but not broken, cells. The insulin-like effects of thiols were related to their content of sulfhydryl residues. The dose-response relationships for the effects of thiols on glucose oxidation and lipolysis were parallel to those for insulin action. The effects of submaximal concentrations of insulin and cysteine together, on glucose metabolism and lipolysis, were additive. Stimulation of glucose metabolism by a supramaximal concentration of insulin (1000 µunits per ml) was not further augmented by the presence of cysteine (0.5 mm) together with the insulin. However, in the presence of 0.56 µm epinephrine, lipolysis was more strongly inhibited by insulin alone (1000 µunits per ml) than by 0.5 mm cysteine together with the insulin. In concentrations greater than 10 mm, thiols suppressed lipolysis to a greater degree than did insulin, in the presence of high concentrations of epinephrine together with caffeine. These concentrations of thiols also profoundly decreased utilization of glucose by both intact and broken cells. The thiols, in the lower concentration range, may share with insulin a common pathway of action. In higher concentrations, the thiols apparently acted at some other site (or sites).