Time course and duration and physical dependence was investigated in rats implanted subcutaneously with 3 different types of morphine (M) pellets. Each was formulated according to the method of Gibson and Tingstand , but differed in surface area and hardness. Animals were maintained for 19 days after implantation and physical dependence was assessed every other day. Severity of naloxone (Nx)-induced withdrawal was quantified by the use of a composite symptom score and weight loss. Withdrawal severity was greatest following implantation of a pellet (Type C) of large surface area and low hardness rating, and least following implantation of a pellet (Type A) of small surface area and high hardness rating. Abstinence severity which resulted from implantation of a pellet (Type B) of moderate surface area and low hardness rating was intermediate. When 2 pellets were implanted the difference between Type C and B was amplified. It was concluded that formulation per se was not sufficient for specifying M pellet characteristics.