We studied the effect of apomorphine on the performance of rats, maintained on a free-operant shock-avoidance schedule under normoxic and hypoxic (12, 10, and 8% O2) environments. In a normoxic environment, apomorphine (1, 2, 4, and 8 mg/kg, i.p.) produced stereotyped behaviors and dose-dependent increases in the rats' lever-pressing response rate. In a hypoxic environment, however, the increase of the response rate induced by apomorphine at 1 mg/kg was suppressed by the hypoxic conditions tested. At a dose of 8 mg/kg apomorphine, it was also suppressed by exposure to severe hypoxia (10 and 8% O2). These results suggest that the impairment of avoidance behavior in rats by hypoxia was not always improved by apomorphine, and that the degree of improvement depends on the combined effects of the dosage of the drug injected, the degree of hypoxia, and the type of behavior observed.