We studied the role of the endothelium in diameter changes as a function of flow of the isolated femoral artery of the rabbit (n = 15) perfused and superfused with a physiological salt solution (37 °C). In 10 vessels, diameters were studied before and after exposure to gossypol, an agent that impairs the endothelial function pharmacologically. In 5 of these 10 vessels we added albumin (1.5%) to the perfusion solution. The mean external diameter (±SEM) after equilibration for 60 min at a transmural pressure of 50 cm H<sub>2</sub>O (n = 10) was: 1,426 ± 34 µ M Vessels were then constricted with norepinephrine (1.0–1.5 µ M in the supervision solution) to 70% of the resting diameter, acetylcholine was used to check endothelial function. All vessels constricted as flow was increased (p < 0.001), irrespective of the impairment of the endothelial function by gossypol or the presence of albumin. It is therefore unlikely that the flow-induced constriction results from a ‘wash away’ effect of endothelium-derived relaxing factor (EDRF). To test whether EDRF could still play a role after gossypol, we used hemoglobin (n = 5) to bind EDRF. Flow-dependent constriction was still observed, although the mean diameter was decreased. We conclude that flow-dependent constriction is either mediated via the endothelial cells, but not via EDRF, or that the endothelial cells are not involved.