The effects of chronic treatment with the selective D1 dopamine receptor antagonist SCH 23390 (0.25 mg kg-1 s.c. twice daily for 18 days, 4 days withdrawal on yawning induced by the D2-receptor agonist quinpirole has been investigated. Low doses of quinpirole (20 and 50 micrograms kg-1 s.c.) induced dose-related yawning behaviour in rats. The yawning response to quinpirole was not significantly different in the presence or absence of SCH 23390 pretreatment. However, chronic treatment with SCH 23390 alone significantly suppressed yawning behaviour. The results suggest that chronic D1-receptor blockade with SCH 23390 does not alter the function of the D2-receptor population mediating yawning behaviour. However, after withdrawal it may result in behavioural activation as seen by suppression of yawning behaviour in the present study.