The effect of diltiazem as an adjunct to cardioplegia was examined for cardiac eicosanoid release in 12 patients who received aorto-coronary bypass. The patients were divided into two groups: diltiazem group (No. = 6) (10 mg/L of diltiazem in crystalloid potassium cardioplegia) and control group (No. = 6) (same cardioplegia without diltiazem). The plasma levels of thromboxane B2 and 6 keto PGF1 alpha (metabolites of thromboxane A2 and prostacyclin) in systemic artery and coronary sinus blood were measured during early reperfusion. A significantly larger arterio-venous difference in 6 keto PGF1 alpha concentration was found immediately after aortic clamp release in the diltiazem group (124 +/- 77 vs 12 +/- 47 pg/ml, p less than 0.05). The plasma thromboxane B2/6 keto PGF1 alpha ratio in coronary sinus blood was lower in the diltiazem group immediately after aortic clamp release (0.47 +/- 0.16 vs 0.74 +/- 0.18, p less than 0.05). There was no significant difference in arterio-venous TxB2 concentration between the two groups. In terms of myocardial protective effect evaluated by CPK-MB release, there was no significant difference between the two groups. The results indicated that diltiazem used as an adjunct to cardioplegia enhanced the production of endogenous PGI2 during early reperfusion, although its beneficial effect on myocardial protection was not shown in terms of enzyme leakage.