1. The role of glutathione content and glutathione S-transferase activity in vascular relaxant responses to nitroglycerin was evaluated in potassium (30 mM)-contracted coronary artery strips of the pig by measuring changes in tension, glutathione content and glutathione S-transferase activity. 2. Prior exposure of coronary artery strips to nitroglycerin (10(-5)M or 10(-4)M for 20 min) resulted in tachyphylaxis to subsequent relaxation to nitroglycerin (10(-8)-10(-5)M). 3. The glutathione content and glutathione S-transferase activity of the arterial strips rendered tachyphylactic by prior exposure to nitroglycerin (10(-5)M for 20 min or 10(-3)M for 120 min) were not significantly different from those of control strips. 4. Treatment with diethyl maleate (10(-4)M or 10(-3)M for 60 min) markedly depleted arterial glutathione content in a concentration-dependent manner with no change in glutathione S-transferase activity. 5. The relaxant response of coronary artery strips to nitroglycerin (10(-8)-10(-5)M) was completely unaffected following treatment with diethyl maleate (10(-4)M or 10(-3)M for 60 min). 6. The results suggest that vascular glutathione content does not play an important role in vascular relaxation or tolerance development to nitroglycerin, at least in pig isolated coronary artery.