Objective: Platelet activating factor (PAF) is a potent mediator in inflammatory responses and maybe involved in various disease states. Degradation of PAF in plasma results from the action of a specific, lipoprotein associated, acetylhydrolase. The aim was to determine plasma acetylhydrolase activity under optimised conditions, PAF half life, phospholipase A2 activity, the lyso-derivative of PAF (lyso-PAF), and lipids in patients undergoing coronary artery bypass grafting. Methods: The study variables were determined 3 d and 7 d following coronary artery surgery and compared to presurgical values in 15 males, age 55(SEM 4) years. Results: Three days following coronary bypass grafting, total, LDL and HDL cholesterol fell significantly by 30%, 45%, and 15% respectively (p<0.001), all decreases correlating with bypass time (p<0.025). Concentrations remained low at 7 d (p<0.005). Acetylhydrolase activity fell by 38% (p<0.001) at 3 d post-surgery and remained depressed, but plasma PAF half life did not change after surgery. The inverse relationship between acetylhydrolase activity and plasma PAF half life preoperatively (p<0.01) was not evident after surgery. There was a direct linear relationship between acetylhydrolase activity and both total (p<0.002) and LDL cholesterol (p<0.001) before surgery. The fall in acetylhydrolase activity correlated with the fall in these lipids (p<0.01) but not with that of HDL cholesterol. Plasma lyso-PAF decreased by 65% (p<0.001) at 3 d and remained depressed (p<0.001). Plasma phospholipase A2 activity increased by 60% (p<0.01) and remained raised (p<0.05), the increase at 3 d being related to bypass time (p<0.05). Conclusions: The large fall in plasma acetylhydrolase activity after coronary bypass grafting is consistent with the fall in plasma lipids. However, the absence of a significant change in the measured PAF half life in plasma raises questions as to the pathophysiological significance of the decrease in acetylhydrolase activity.