1. Renal cellophane wrapping to produce hypertension causes thickening of the capsule of the kidney. To determine whether this compresses the kidney, deep renal vein wedge pressure was measured as an estimate of tissue pressure in anaesthetized rabbits 1 month after cellophane wrapping (n = 5) or a sham operation (n = 3). 2. Renal vein wedge pressure was 18.3 +/- 2.0 mmHg in hypertensive rabbits and 8.4 +/- 1.1 mmHg in the sham-operated rabbits. 3. Arterial pressure was raised or lowered with angiotensin II or glyceryl trinitrate, respectively. Arterial and wedge pressures were approximately linearly related and, at any given arterial pressure, wedge pressure was approximately 8 mmHg higher in the cellophane-wrapped kidney than in the kidney of the sham-operated group. 4. These results, showing that renal wedge pressure is elevated in renal wrap rabbits, indicate that the kidneys are compressed, probably by the thickened renal capsule. This may explain the increased renal vascular resistance seen in this form of hypertension.