The effect of hyperosmolal glucose infusion on the transcapillary transport of fluid and protein was examined in the isolated gracilis muscle of the dog. Volumes of 18, 36, and 72 microliter/min of 30% glucose were infused intraarterially for 30 min into blood flow held constant at 3-4 ml/min X 100 g, thereby increasing plasma osmolality from 21 to 126 mOsm/liter. Transcapillary fluid movement (Jv) was assessed plethysmographically and protein transport (Js) by direct monitoring of the rate of increase of tissue 125I-albumin radioactivity. Hyperosmolal perfusion reduced vascular resistance. Initially tissue volume declined and then while the infusion continued, reversed direction and rose, occasionally exceeding control volume within the 30-min infusion period. Js also declined transiently, then increased during infusion. The reversal of Js led the reversal of Jv, therefore the reversal of Jv during hyperosmolal infusion was attributed to development of a sufficient change in the oncotic gradient due to the increased Js. These changes were significantly dose dependent. Stopping the hyperosmolal infusion resulted in a marked increase in Jv at all dose levels and, except for the low dose, a maintained rate of increase of Js. Thus early during hyperosmolal exposure protein transport was enhanced by either diffusion or vesicular transport. During recovery, the maintained increase in Js was probably due to increased convection.