In the rat, cardiac muscle extends into the pulmonary vein. To date, only one study (MacLeod & Hunter, 1967) has described the influence of acetylcholine (ACh) upon the contractile response of this muscle. It was shown qualitatively that the paced contractile response was attenuated by application of exogenous ACh. There were two aims in the present study. The first was to quantitatively assess the concentration dependence of the ACh-mediated attenuation of the paced contractile response of the pulmonary vein in the rat. The second aim was to determine whether the ACh effect was mediated by the endocardium of the vessel.Male Wistar rats (265-320 g body mass) were killed by a stunning blow to the head followed by cervical dislocation. A pulmonary vein was dissected free post-mortem and mounted in a myograph for assessment of isometric tension development. The vein was superfused with Tyrode’s solution bubbled with oxygen (100%) at 37°C and set to a pre-tension equivalent to a transmural pressure of 15mmHg. The vein was electrically paced (1msec pulse width, 1 Hz, 10V). In one experimental series the contractile responses were then determined in the presence of ACh (1 nM-10µM). In the second series the influence of a single application of ACh (1µM) was determined before and after endocardial abrasion by a human scalp hair.In all experiments (n=8) ACh attenuated the paced contractile response. The attenuation was concentration-dependent (n = 4, figure 1). In the second experimental series (n=4) the attenuation (%) effected by 1µM ACh (median 30, upper quartile 42, lower quartile 8) post-abrasion was not significantly different (Mann-Whitney U test, P=0.49) to the attenuation prior to abrasion (median 42, upper quartile 46, lower quartile 20).This study has demonstrated that ACh attenuates the paced contractile response of rat pulmonary veins in a concentration-dependent manner, and that an intact endocardium is not essential for this phenomenon.