The transcapillary escape rate of albumin was studied in 24 juvenile hypertensives and in 13 controls on the basis of the radioactivities determined in the blood during 60 min after i.v. injection of human 131I-labelled albumin. In addition, the central haemodynamics of all hypertensive subjects was examined at rest and during exercise. The patients were grouped according to their pressure during the investigation at rest. The group whose mean arterial pressure did not exceed 100 mmHg, thus being within the range of normotension, did not exhibit a significant deviation from controls. The group with mean arterial pressure exceeding this limit (corresponding to the range of borderline hypertension) showed a significantly higher escape rate of albumin (9.3 +/- 5.0%/h) than controls (5.2 +/- 1.6%/h). The albumin escape rate was significantly correlated to the mean pulmonary artery pressure during exercise but not to the remaining central haemodynamic parameters at rest or during exercise. No difference was seen in the magnitude of plasma volumes of juvenile hypertensives and controls. The findings suggest that secondary changes of capillary functions are present already in the early stages of hypertension.