1. A method of continuous infusion in the unrestrained rat is described, which provides a scientifically acceptable and easily maintained rodent model for use in toxicological investigations. 2. Sprague Dawley SPF rats had cannulas implanted into the vena cava via the femoral vein, and were continuously infused with physiological saline for a total of 28 or 90 days. 3. The results indicate that there was no change in body weight, food consumption, clinical observations or clinical biochemistry of infused rats when compared to non-infused rats. There were small changes in haematological parameters, however none were toxicologically significant. Urinary volume was increased and urinary specific gravity and osmolality were decreased. At macroscopic and microscopic examination there were findings of scar formation associated with the area of surgery and minimal irritation in the area of the vena cava which accommodated the cannula. 4. These results indicate that implantation of a cannula into the vena cava of a rat and subsequent continuous intravenous infusion of physiological saline produces no toxicological adverse effects over a period of 90 days. Consequently, this model can be recommended for the continuous intravenous administration of test substances to rats.