Homothallic Saccharomyces cerevisiae, growing exponentially in a synthetic acetate medium, could be initiated to undergo meiosis and subsequent sporulation by removal of sulfur from the medium or by partial purine deprivation of purine auxotrophs or, most efficiently, by guanine deprivation of a guanine auxotroph. In contrast, partial uracil deprivation of uracil auxotrophs did not cause sporulation. Under any of the above and other sporulation conditions, the intracellular concentrations of GTP and, usually at some time later, S-adenosylmethionine (SAM) decreased; the concentrations of the other nucleoside triphosphates decreased under some but increased under other sporulation conditions. The addition of 1 mM methionine or, more effectively, of SAM or the combination of adenine plus methionine greatly increased the intracellular concentration of SAM and reduced or prevented sporulation, even when GTP decreased. However, differentiation can be inhibited by an excess of many metabolites which do not specifically control the initiation process; in particular, SAM is known to inhibit yeast metabolism (e.g., transamination). Therefore, we cannot yet decide whether the deficiency of GTP or SAM (or related compounds) serves as a signal for the initiation of meiosis/sporulation.