The application of radio-iodinated human serum albumin (RISA) to the measurement of a continuously changing plasma volume, such as that occurring during heat exposure and exercise, has been considered in terms of the exchange dynamics of albumin between the intravascular and extravascular compartments. In six male subjects resting supine for 2 h in a hot environment, or exercising for 50 min in a thermoneutral or hot environment, there was no statistically significant alteration in the rate of protein efflux from the intravascular space. However, following exercise, protein was added to the circulation at a greater rate than it was lost through the capillary walls. A technique for calculating plasma volume from a single measurement of plasma RISA activity is described. This may be used in conjunction with measurements of changes in haemoglobin concentration of determining plasma volume in situations where alterations in protein exchange dynamics do occur.