The platelet response to the aggregatory effect of platelet-activating factor (PAF) in relation to blood PAF levels, serum PAF-acetylhydrolase (PAF-AH) activity and to their lipidaemic profile, was studied in 44 patients with coronary artery disease undergoing exercise tests. The PAF EC50 values in 21 patients with positive exercise test results were found to be significantly decreased at rest compared with 21 normal subjects (12.6 +/- 3.9 nM and 24.9 +/- 11.7 nM respectively) (P<0.0001). Moreover, the maximal percentage of aggregation to 50 nM PAF was found to be significantly increased (20.0 +/- 4.3% vs 13.5 +/- 3.6%, respectively) (P<0.0001). By contrast, the PAF EC50 values and the maximal percentage of aggregation in 23 patients with negative exercise test results were not statistically significantly different from the control group (25.2 +/- 11.4 nM and 14.1 +/- 4.7%, respectively). At the end of exercise, the PAF EC50 values and the maximal percentage of aggregation did not change in any group, and there were no significant differences in the whole-blood PAF levels either at rest or at the end of exercise. In patients with positive exercise test results, the PAF-AH activity at rest was significantly higher compared with the control group (37.2 +/- 8.0 nmol.ml(-1).min(-1) vs 32.4 nmol.ml(-1).min(-1), (P<0.03), whereas the enzyme activity did not differ in patients with negative exercise test results compared to controls (33.6 +/- 6.1 nmol.ml(-1).min(-1)). There was no change in PAF-AH activity during exercise in any group. The enzyme activity was positively correlated to the serum total and low density lipoprotein (LDL) cholesterol levels in the control group and in patients with negative exercise test results, whereas no correlation was found between PAF-AH activity and total or LDL cholesterol levels in patients with positive exercise test results.