To test the hypothesis that drug entry site (intimal vs. adventitial surface) may be an important determinant of methylene blue (MB) action on vascular reactivity to contractile stimulation, we used preparations of rat isolated tail arteries perfused at constant flow (2 ml/min). Perfusion pressure changes reflecting vascular responses to cumulative addition of phenylephrine (PE) or dopamine (DOP) were recorded before and after MB addition (2.5 x 10(-4) M final concentration) at the intimal or adventitial surface. Intraluminal addition of MB resulted in strong potentiation of subsequent responses to intimal contractile stimulation using PE or DOP. This effect was not observed when MB was added at the adventitial surface. These results suggest that drug entry site is an important determinant of MB action on vascular responsiveness in the rat tail artery and might account for the divergent results in the literature on the vascular action of MB.