Comparison of plasma levels of ACTH (pg/ml), corticosterone (CORT, ng/ml) and prolactin (ng/ml). Four groups of animals: (1) H-RI, the heroin SA rats with “high” reinstatement induced by acute foot shock stress; (2) L-RI, the heroin SA rats with “low” reinstatement induced by acute foot shock stress; (3) NR, the rats below the criterion for acquisition of heroin SA (20 infusions on days 5–7 of SA testing, 0.05 mg/kg/infusion); and (4) Stress control, saline SA rats without acute foot shock stress (n = 6–10). Shown are the mean levels measured 45 min after 15-min intermittent foot shock stress on day 16 (9 days after heroin SA), and in the Stress control group.