Tolerance to and physical dependence on morphine in the rat was induced by injecting increasing doses of morphine sulfate (M.S.) administered i.p. twice daily for 14 days. The last dose of M.S. was 200 mg/kg. This procedure produced a 4-fold tolerance to morphine as evidence by the increased dose of morphine required to produce analgesia. The degree of dependence was quantified by determining the naloxone ED50 for the stereotyped withdrawal jumping response. Body weight loss and hypothermic responses during abrupt and naloxone-induced withdrawal were also measured. The degree of tolerance and dependence produced by multiple injection procedure was comparable to that produced by 2 pellets containing 75 mg of morphine base implanted for 3 days. The level and turnover of brain serotonin, determined 6 or 12 h after the last morphine sulfate injection did not differ significantly from that of saline injected control animals. These data indicate that multiple injection technique produces a mild degree of tolerance to, and physical dependence on, morphine which was not related to changes in brain serotonin level or turnover.