Interstitial fluid pressure (IFP) was measured by the wick method in six young men. During exercise in the heat, IFP progressively decreased, and this decrease was maintained after cessation of exercise. In two men working in the same conditions after heat acclimatisation IFP became initially positive before steadily declined. In three anaesthetised dogs exposed to heat IFP also declined, but in a thermoneutral environment it rose towards atmospheric pressure. It is suggested that IFP could be used as a measure of filtration forces operative across the capillary wall. Fluid dynamics in the interstitial space of heat acclimatised man differed, however, from that of unacclimatised man and heat exposed dogs. It is postulated that these changes may be related to movement of protein from the interstitial to the plasma compartments during exercise after acclimatisation...