Experiments were designed to determine whether insulin-dependent diabetes mellitus (IDDM) alters direct chronotropic effects of adrenergic and cholinergic agonists and whether the observed changes are associated with hyperglycemia or combined hyperglycemia and ketoacidosis. Diabetes was induced by intravenous administration of 45, 50, or 65 mg/kg streptozotocin (STZ). Rats treated with 65 mg/kg STZ had higher levels of blood glucose and ketones compared with the levels of the other groups. Right atria were isolated 12 wk after administration of STZ and bathed in Krebs-Henseleit solution. Basal spontaneous pacemaker rate was diminished in preparations isolated from diabetic rats. The maximum pacemaker rate observed during exposure to isoproterenol or norepinephrine was also depressed in preparations from diabetic animals; however, the increase in rate and half-maximal effective concentration values for each agent were not affected. The sensitivity to the negative chronotropic action of acetylcholine was enhanced by IDDM, whereas the response to carbachol (a cholinergic agonist not readily metabolized by acetylcholinesterase) was not changed. No significant differences were observed when we compared preparations isolated from diabetic animals with and without ketoacidosis. In summary, these data suggest 1) that IDDM is associated with a diminished basal spontaneous pacemaker without changes in the responsiveness to adrenergic and cholinergic receptor activation and 2) that ketoacidosis does not play a role in the observed alterations.