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In this study, the distributions of calcitonin gene-related peptide, neuropeptide Y, and α-atrial natriuretic peptide 1–28 immunoreactivity, were investigated within different regions of the guinea pig heart by utilising two different methods of tissue fixation for the immunocytochemistry. The results were compared with data obtained through radioimmunoassays. We observed similar concentrations and distributions of α-atrial natriuretic peptide in the right atrium, with results of radioimmunoassay and immunocytochemistry, but there were no myocytes containing α-atrial natriuretic peptide in the left atrium or ventricles with immunocytochemistry as opposed to radioimmunoassay. The immunoreaction obtained for neuropeptide Y was more intense in the right ventricle than left. Calcitonin gene-related peptide nerve fibres were about twice as abundant in the left atrium than in the right.