Modified needle oxygen microelectrodes and vital microscopy were used to measure transmural oxygen tension gradients (PO2) in pial arterioles with lumen diameters of 20-90 microm. A relationship between the magnitude of the transmural PO2 gradient and arteriole wall tone was found: in control conditions, PO2 gradients were 1.17 +/- 0.06 mmHg/microm (n = 40), while in conditions of arteriolar wall dilation the transmural PO2 gradient decreased to 0.68 +/- 0.04 mmHg/microm (p < 0.001, n = 38). These data provide the first measurements of transmural PO2 gradients in pial arterioles of different calibers at different levels of vascular tone and have fundamental importance for assessing the role of arterial microvessels in tissue oxygen supply processes. The results obtained here provide evidence that oxygen consumption by the vessel wall is within the range characteristic of enveloping tissues and that oxygen consumption by the endothelial cell layer probably has no significant effect on the magnitude of the transmural PO2 gradient.