1. The involvement of nitric oxide (NO) and platelet activating factor (PAF) in the systemic depressor responses to increased renal perfusion pressure (RPP) were investigated. 2. In anaesthetized rabbits, the left kidney was perfused via an extracorporeal circuit which allowed RPP to be increased from 65 mmHg to 125 mmHg. The response of systemic blood pressure (SBP) to increasing RPP was measured in the same rabbits. 3. One group of rabbits (n = 5) was treated with NG-nitro-L-arginine (NOLA) to inhibit NO synthase activity (20 mg/kg i.v. bolus). Another group (n = 5), received 250 mmol/L NaHCO3 (4 mL/kg bolus) as vehicle treatment. 4. Following an increase in RPP to 125 mmHg, SBP fell at a rate of 0.43 +/- 0.06 mmHg/min in the vehicle treated rabbits. After NO synthase inhibition the rate of fall in SBP of 0.34 +/- 0.07 mmHg/min was not significantly different from that in the vehicle group (P = 0.3). 5. Blockade of NO synthesis did not alter the renal blood flow, renal vascular resistance changes and pressure-related natriuresis and diuresis responses to increased RPP to 125 mmHg. 6. PAF receptor blockade, using WEB 2086 (0.5 mg/kg plus 0.5 mg/kg/h), did not alter the systemic, renal haemodynamic or urinary responses to increasing renal perfusion pressure to 125 mmHg. 7. These findings indicate that neither NO nor PAF play an important role in the blood pressure lowering activity, intrarenal haemodynamics and urinary excretory responses observed when RPP was increased to a level within the physiological range.