Rhesus monkeys implanted with intravenous catheters readily self-inject a wide variety of psychoactive drugs. Under conditions where access to drug is not limited, pharmacological variables are important determinants of the rate and pattern of drug-reinforced responding. Data for unlimited access phencyclidine self-administration are presented to illustrate the types of findings obtained from unlimited access studies. The results with phencyclidine indicate a pattern of drug intake that progressed from variable day-to-day response rates at a low dose to relatively stable daily response rate with greater drug intake at a higher dose. The pattern of responding, toxicity, and development of physical dependence seen in these subjects were similar in many respects to results found with several other drugs under conditions of unlimited access. These findings demonstrate that unlimited access studies are particularly useful for revealing toxic consequences of self-administered doses. Moreover, a review of the literature indicated that procedures allowing unlimited access by drug-naive monkeys were at least as likely to reveal reinforcing properties of drugs as limited access procedure using drug-experienced subjects.