Platelet activity is controlled, in part, by cytosolic free ionized calcium concentration ([Ca ++] i). Regulation of platelet thromboxane (TXB 2) synthesis may be by regulation of [Ca ++] i. Dietary linoleate is a regulator of TXB 2 synthesis, therefore, it may act by influencing [Ca ++] i. Aspirin is a regulator of TXB 2 synthesis by inhibition of cyclooxygenase; ouabain and nifedipine are regulators of [Ca ++] i. This study was conducted to determine whether these affectors of TXB 2 synthesis and [Ca ++] i cause associated responses. Male nonobese Zucker rats were fed diets supplying 30% of energy (en%) as fat. Dietary fat was a mixture of corn oil and beef tallow to provide 3.0, 4.5, 6.0, or 7.5 en% linoleic acid, with cholesterol added to provide equal cholesterol in all diets. Rats were fed for 30 days with 6 rats/diet. Isolated rat platelets were assayed for FA composition; the percentage of linoleic acid in platelet FA rose linearly with increasing dietary linoleate (r = 0.76, P < 0.0001). Resting and thrombin-stimulated platelet [Ca ++] i and TXB 2 synthesis were measured in the presence or absence of extracellular calcium and aspirin, ouabain, or nifedipine. Aspirin caused reductions in both parameters; nifedipine blocked [Ca ++] i, but did not affect TXB 2; ouabain increased both. Changes induced by those modifiers of TXB 2 and platelet [Ca ++] i caused changes that were in the same direction for both. CaCl 2 caused an increase in both and the [Ca ++] i was correlated with the square root of the TXB 2; without CaCl 2 the two were negatively correlated; aspirin, ouabain, and nifedipine treatments resulted in no significant correlations. The results suggest that there is a common modifier of [Ca ++] i and TXB 2 synthesis.