The characteristic actions of pentazocine in morphine-dependent rats were investigated by a drug-admixed food (DAF) method. Pentazocine did not cause evident withdrawal signs when stopped after a continuous administration for 2 months at 3 different dose levels. A state of physical dependency on morphine was produced in rats by feeding them for 3 weeks with food that contained different levels of morphine. Animals exhibiting a moderate degree of morphine-withdrawal signs (17-18 hr after withdrawal) received a s.c. cross-administration with pentazocine at 0, 5, 10, 20, 40, 80, and 150 mg/kg. This drug at 20 mg/kg proved most supressive of morphine-withdrawal signs, being about 1/4 as potent as codeine. In a dose range from 20 to 40 mg/kg, the action of pentazocine on the withdrawal signs was reserved, that is, doses not less than 40 mg/kg exerted a dose-related antagonistic action in all the 3 levels of morphine dependent rats. The challenge with levallorphan (2 mg/kg, s.c.) during the chronic application of pentazocine has biphasic action on morphine dependence and suggests that this type of drug must have different properties from morphine type drugs.