Twenty dogs (weight 8-14 kg) were anaesthetized with sodium pentobarbitone 25 mg per kg body weight and anaesthesia was maintained with nitrous oxide in oxygen. Respiration was controlled with a volume-limited respirator (PaCO2 4.7 +/- 0.4 KPa). Renal blood flow was measured with a square wave electromagnetic flow meter inserted through a left paramedian laparotomy. Renal blood flow decreased transiently following an i.v. injection of pentazocine. The decreases were 81% +/- 10.8 and 77% +/- 3 of the control following the administration of pentazocine 1 mg and 2 mg per kg body weight respectively (P less than 0.01). Renal blood flow returned to control values 10-15 min after the injection of the drug.