The relation between availability of metabolic energy and thrombin-induced platelet aggregation and secretion was investigated in a system of transient substrate depletion followed by restoration of ATP resynthesis. Substrate depletion induced a fall in the concentration of metabolic ATP and in the adenylate energy charge and a concurrent decline in aggregation and secretion of dense and alpha-granule contents. Restoration of energy generation completely restored the adenylate energy charge and restored aggregation and secretion, but led to incomplete recovery of the ATP concentration. A close correlation between the adenylate energy charge and aggregation and between the adenylate energy charge and the secretion of dense and alpha-granule contents could be demonstrated. No such correlation existed between these responses and the concentration of ATP. These results show that the adenylate energy charge monitors an energetic condition which is crucial for preservation of platelet aggregation and secretion of dense and alpha-granule contents.