1. The A23187-induced secretion of preabsorbed serotonin from human blood platelets at 37 degrees C is studied. Preincubation at the same temperature before the addition of ionophore is necessary for maximal release induction. When total incubation time is kept constant, longer time with ionophore results in a smaller decrease in the level of metabolic ATP and increase in metabolic ATP and increase in metabolic IMP. This coincides with the reduction in secretion, but statistical treatment of the results suggests that the reduced secretion only partially explains the reduced drop in metabolic ATP, and that therefore a resynthesis of metabolic ATP from IMP may have taken place. 2. In some experiments induction of secretion takes place over a very narrow range of ionophore concentration. 3. When K+ substitutes for Na+ in the extracellular medium, the need for preincubation for maximal secretion becomes less evident, and at times is abolished, while there is still a significant increase in the metabolic ATP level by prolonged incubation with ionophore. 4. A reduction in secretion is observed with metabolic blockers when the ionophore is added after preincubation, but to a much less degree than when secretion is induced by thrombin, in spite of a great reduction in the level of metabolic ATP. This may partly be explained by the increase in secretion induction by A23187 in the presence of inhibitors when the ionophore is added in the cold, suggesting that the inhibitor may cause "weakening" of the platelets' "resistance" to induction of secretion by ionophore. 5. When the effect of Ca2+ and of Mg2+ on the level of intermediates of the TP leads to hypoxanthine conversion is studied, it is evident that the addition of Ca2+ causes enhanced IMP accumulation and a reduction in the level of inosine plus hypoxanthine, while Mg2+ has the opposite effect. This suggests that the two metals affect the enzymes of the IMP leads to hypoxanthine conversion differently. 6. Indomethacin inhibits secretion induced by A23187, suggesting that prostaglandin intermediates may amplify the ionophore-induced release. The adenine nucleotide metabolism is not affected. 7. The results indicate that there is an indirect, rather than direct, link between the major metabolic changes and the secretion induced by A23187, but that the ionophore may cause intracellular changes which are not connected to its effect as release inducer.