A method is described for obtaining potency estimates of narcotic analgesics to morphine in experimental addict rats. Drugs were administered through a chronic right heart cannula and intake was under the rat's voluntary control. Animals were offered two doses of morphine (3.2 and 10 mg/kg/injection) and two doses of test compound on a fixed ratio reinforcement schedule of 10:1. Logarithm of the daily number of injections taken was used as the effect metameter. Each potency estimate was based on results in four rats. Rats averaged 137 mg/kg/day of morphine when offered 10 mg/kg/injection. A diurnal variation in opiate intake was observed, the nighttime rate being about one-third greater than the daytime rate. Morphine intake measured before and after the assay period was essentially unchanged. Potency estimates and the 95% fiducial interval were: dihydromorphinone = 10 (5.2−19) × morphine, methadone = 3.4 (2.7−4.6) × morphine and codeine = 0.67 (0.45−1.0) × morphine. Analgesic activity for codeine was not proportional to its ability to substitute for morphine in addict rats.