The chronotropic response of isolated right atria obtained from rats made diabetic 14-15 weeks previously by streptozotocin, was compared with age-matched controls. Diabetic rat atria are significantly more sensitive to the negative chronotropic actions of adenosine and of methacholine. Pretreating both control and diabetic rats with 2.5 mg kg-1 pertussis toxin attenuated the negative chronotropic effects of methacholine and adenosine on isolated atria, although diabetic atria still displayed a significantly greater sensitivity to these agonists (P less than 0.05-0.001). The negative chronotropic effects of methacholine and adenosine on both control and diabetic atria were abolished following pretreatment with higher doses of pertussis toxin (10 mg kg-1). These results suggest that pertussis toxin-sensitive G proteins may be involved in the supersensitivity of diabetic hearts to methacholine and adenosine.