The pressure within rat cardiac and renal allografts has been observed to rise during rejection. We wished to see if this pressure rise could be prevented or reversed with immunosuppression. Transplants were performed from Lewis or DA donors to Lewis recipients. The rats received either a heterotopic cardiac transplant or an orthotopic renal transplant and were then treated with different immunosuppressive protocols. Intramyocardial pressure was recorded using a fine-gauge needle connected to an air pressure manometer. In the case of the renal transplants, a pressure transducer was used as well and the two methods compared. Intramyocardial and intrarenal pressures rose dramatically in unimmunosuppressed recipients of DA allografts. No such rise was seen in isografted organs, although the pressures recorded remained significantly higher than those found in untransplanted hearts and kidneys. Cyclosporine 20 mg/kg/day was effective in suppressing rejection in both models, and inhibited any rise in intraorgan pressure. Cyclosporine 10 mg/kg/day was less effective, and with 2 mg/kg/day allograft function was considerably impaired, one-third of the cardiac grafts being rejected by 16 days. In both models intraorgan pressures became raised. The addition of methylprednisolone 16 mg/kg i.p. on days 7 and 8 to this low dose regimen of cyclosporine 2 mg/kg/day rapidly reversed the rise in pressure and restored graft function to normal. Intraorgan pressure levels therefore accurately reflected the state of function of transplanted hearts and kidneys. When a manometer and a transducer were compared as a means of measuring the pressure in the renal transplants, the manometer method was found to be superior.