Adenylate cyclase activity and endogenous cyclic AMP levels were measured using a highly sensitive radioimmunoassay and protein binding assay during 24 h of development of Dictyostelium discoideum. Adenylate cyclase activity was not detected until the aggregation stage of development (10 h) when a sudden peak of activity was found. The enzyme was active at all subsequent stages, although a slow decline in activity was observed. Similarly, cyclic AMP levels were not detectable through the first 7 h of development and then showed a sudden peak at aggregation. Following aggregation the cyclic AMP levels decreased to approximately 1/2 the peak value and maintained that level throughout the remainder of the developmental cycle. Adenylate cyclase had a narrow range of substrate saturation with a maximum velocity at 1 to 4 mM ATP at both the aggregation stage (10 h) and the sorocarp stage (24 h). At levels of ATP higher than 6 mM the enzyme from both stages was strongly inhibited. No activity was observed in the absence of Mg2+ or dithiothreitol. The activity from 10-, 14-, and 20-h stages was found bound to a 25,000 x g pellet fraction. The sudden appearance of adenylate cyclase and its product cyclic AMP at aggregation provides additional evidence of a role for this nucleotide in chemotaxis, and the retention of enzyme activity and nucleotide level during the subsequent stages may reflect a further function of cyclic AMP during formation of the two cell types.