Pigeons trained to discriminate morphine from saline under a color tracking procedure received cumulative doses of cyclazocine after various regimens of daily morphine or saline administration. Cyclazocine generalization curves were obtained after zero, one, two, or six completely drug-free days. In four or five animals, cyclazocine produced more response generalization to morphine after six drug free days than after no drug free days. In two animals the cyclazocine dose-effect generalization curves were generally shifted to the left with increased time from last drug exposure. Morphine response generalization to cyclazocine was also related to the degree of stimulus control evident in the non-drug (saline) condition during early portions of the subjects' experimental histories. The less stimulus control evident in the non-drug (saline) condition (i.e., the more morphine-appropriate responses made after saline injections and the greater variability of such responding), the more likely it was to obtain morphine-appropriate responding after cyclazocine administration.