This study explored physiological mechanisms of diabetic dysfunction in baroreceptors and chemoreceptors-mediated hemodynamic responses, and cholinergic neurotransmission in 30-day diabetic rats (n = 14) and controls (n = 14). Basal hemodynamic data and vagal response to electrical stimulation and methacholine injection were also evaluated. Muscarinic receptors were characterized using a radioligand receptor binding assay ([3H]N methylscopolamine). Experimental diabetes (50 mg/kg of STZ, i.v.) decreased systolic, diastolic, and mean arterial pressure and basal heart rate. Heart rate (HR) responses to vagal electrical stimulation (16, 32, and 64 Hz) were 15%, 11%, and 14% higher in diabetics vs non-diabetics, as were HR responses to methacholine injection (-130+/-24, -172+/-18, -206+/-15 bpm vs. -48+/-15, -116+/-12, -151+/-18 bpm, P < 0.05). Muscarinic receptor density was higher (267.4+/-11 vs 193.5+/-22 fmol/mg/prot, P < 0.05) in the atria of diabetic rats than in those of controls; the affinity was similar between groups. Diabetes-induced reduction of reflex responses to baro- (reflex bradycardia: -3.4+/-0.3 and -2.7+/-0.2 bpm/mm Hg; reflex tachycardia: -1.6+/-0.1 and -1.4+/-0.07 bpm/mm Hg, in control and diabetics, P < 0.05) and chemoreceptor stimulation, enhancement of HR responsiveness to cardiac vagal electrical stimulation and methacholine stimulation, plus an increase in the number of atrial muscarinic receptors indicates reduced parasympathetic activity, which is probably derived from central nervous system derangement.