A study in anaesthetised dogs aged 2, 6, 13, 26 and 52 weeks or older was done to determine the dose of IV bupivacaine given by infusion required to produce severe cardiovascular depression as measured by a drop in mean blood pressure to about 25 mmHg or the development of life-threatening dysrrhythmias. Plasma local anaesthetic concentration was analysed at the time of ceasing the infusion. The toxic dose was significantly less in the six-week-old group than younger or older animals. The plasma concentrations associated with toxicity in the groups 13 weeks and younger were significantly lower than in the older dogs. Treatment of the induced toxicity included hyperventilation, adrenaline infusion, bretylium and/or cardioversion. The response to treatment was better in young dogs than adult dogs.