The effects of various compounds on the time course of the coronary dilating response to nitrovasodilators were studied in working rat hearts. Continuous administration of 100 microM glyceryl trinitrate (GTN) to the perfusate induced a rapid increase in coronary flow. The flow decreased to about 40% of the initial flow rise within 20 min and remained constant during the following 40 min. The flow increase induced by 100 microM SIN 1 (3-morpholino-sydnonimine), however, remained constant throughout the 60-min perfusion period. The decay of the GTN action was reduced in the presence of the thiol-containing agents L-homocysteine, L- and D-cysteine and to a lesser extent, reduced glutathione, while the initial response to GTN was only slightly enhanced. However, N-acetyl-L-cysteine caused a significant increase of the maximum response to GTN whereas the subsequent loss of GTN action was not greatly altered, suggesting that this agent exerts a potentiating action without altering tolerance. Dithiothreitol elicited a slight increase in the maximum effect of GTN, followed by a complete attenuation of the effect. These data suggest that SH group donors could exert different effects on the action of GTN while GTN tolerance is only influenced by L- and D-cysteine, L-homocysteine and reduced glutathione.