Albumin clearance and blood flow were measured in normal tissues and in fibrosarcomas by means of a multiple tracer method in intact male rats. Sarcomas implanted in testes had higher albumin clearances than the same tumours implanted in skeletal muscle, 59 and 26 microliters min-1 per 100 g, respectively. Albumin extraction (defined as the ratio of albumin clearance to plasma flow) was found to be 1 x 10(-4) - 14 x 10(-4) in normal tissues. Fibrosarcomas in muscle showed high extractions (24 x 10(-4)), especially in the central portions (96 x 10(-4)), where the extraction was similar to that in tumours implanted in the testes (111 x 10(-4)). The high albumin extraction in poorly perfused central parts of the muscle-implanted tumours and in the similarly insufficiently perfused testicular sarcomas is probably due to an increased endothelial permeability of the exchange vessels and not to an increased capillary exchange surface area compared with the better-perfused peripheral parts of the muscle-implanted tumours. The latter tissue also had a high albumin clearance, however, in the range of that of maximally dilated skeletal muscle vasculature, possibly reflecting a large functional capillary surface area.