There is evidence that autonomic nerves are disturbed in spontaneous diabetic BB rats indicating a peripheral diabetic autonomic neuropathy. For histofluorescent visualization of catecholamine containing nerve fibres in myocardium the method of De La Torre was applied. For electron microscopy, tissue specimens of ventricular and atrial myocardium were prepared. In myocardium of BB rats diabetic for 28 weeks morphometric quantification of catecholamine histofluorescence revealed a loss of fluorescent nerve fibres and varicosities of 57% from 1.18 +/- 0.17 to 0.51 +/- 0.18 per cent of the total field area (p < 0.009). Ultrastructurally, morphometric quantification of the density of axon profiles containing synaptic vesicles in transverse sectioned atrial myocardium showed a decrease of 42% (p < 0.021) in the diabetic group. In the diabetic nerves, axoplasmic lysis and a reduction in the Schwann cell envelope were prominent. Morphologic changes of nerves in ventricles were comparable to the findings in atrial tissue. It is concluded that a diabetic autonomic neuropathy of intramural sympathetic nervous tissue occurs in myocardium of spontaneous diabetic BB rats despite insulin substitution.