Rhesus monkeys (Macaca mulatta), ranging in weight from 3 to 4.5 kg were employed in this study. The animals had no history of having received any drugs other than antibiotics in the laboratory. The animals were prepared for the intravenous self-administration experiments according to the procedure of Yanagita, et al. (1965). In brief, the monkeys were permitted to have relative freedom of movement in a cage approximately one cubic yard in size. To accomplish this, the monkeys were placed in a light, tubular stainless steel harness which was connected, in turn, to a flexible tubular restraining arm, one end of which was anchored to the cage wall. This arrangement permitted the monkey to move within the confines of the cage freely in all respects except that horizontal rotation was restricted to 300 degrees. The monkeys became accustomed to the harness and the limitations of movement within a matter of a few days. At this time, a permanently indwelling catheter was implanted via the external jugular vein into the right atrium. The catheter was led subcutaneously to a point between the shoulder blades where it emerged through the skin, was led through the tubular restraining arm, and attached to a mechanical injector which was mounted on the outside of the cage. The restriction of rotation imposed by the restraining arm prevented fouling of the catheter by repeated circling movement of the monkey. The mechanical injector consisted of a motor-driven syringe which could be activated either by an electrical timer or by the monkey if it pressed an appropriate lever. After the monkey had recovered from the surgical procedure, and it was determined that the catheter was patent, the lever was placed in a panel in the wall. All monkeys will, out of curiosity, play with any new object which enters their environment and, thereby, activate the injector by pressing the lever. Solutions of nicotine base, dissolved in saline, were delivered to the monkeys