Glutamic acid decarboxylase (GAD) from Neurospora crassa was assayed in dormant and germinating conidia that had been permeabilized by toluene and methanol. N. crassa conidia contained 10 times the GAD activity found in vegetativemycelia. During conidial germination, GAD activity rapidly decreased to low levels before germ tubes appeared. GAD activity in germinating conidia closely followed the decreasing rate of glutamic acid metabolism. Inhibiting protein synthesis partially blocked the decrease in GAD activity, but eliminating exogenous carbon sources did not alter the initial rate of decrease in this enzyme. However, when conidia were incubated for more than 3 h in distilled water, GAD activity began to increase and eventually reached levels comparable to those in dormant conidia. Either GAD was reversibly inactivated or this enzyme could be synthesized from endogenous storage compounds when conidia were incubated in distilled water. These results are consistent with the hypothesis that GAD is a developmentally regulated enzyme that is responsible for catalyzing the first step in the metabolism of the large pool of free glutamic acid during conidial germination.