The time of appearance and distribution of neuropeptide Y (NPY) in the developing and adult rat heart has been studied by immunohistochemical method using the Avidin biotin technique. At 18 days of gestation and postnatal day 1, no immunoreactivity was detected. Immunoreactivity was first observed on postnatal day 5 around the large blood vessels, developing coronary arteries and very sparsely along the blood vessels in the musculature. At later stages up to adulthood the intensity of immunostaining and density of fibres along the intramyocardial blood vessels increased. After postnatal day 10 the NPY immunoreactive fibres were also seen amongst the muscle fibres. Immunopositive NPY neurons were seen in the ganglionic clusters close to the great vessels at postnatal day 20 and were visualised in between the ventricular muscle bundles near the atrioventricular region only in the adult heart. The ventricular musculature was always more richly innervated than the atria by NPY immunopositive nerve fibres. Thus the present study demonstrates that NPY-like immunoreactivity makes its appearance postnatally, first around the blood vessels and later on the muscle fibres too.