The localization and origin of substance P (SP)-, neuropeptide Y (NPY)-, and noradrenaline/tyrosine hydroxylase (NA/TH)- immunoreactive (IR) nerves in the guinea-pig heart were investigated by means of immunohistochemistry; quantitative analysis was performed by radio-immunoassay (NPY) and high performance liquid chromatography (NA). Both untreated animals and animals subjected to stellatectomy, combined stellatectomy and local capsaicin pretreatment of the vagal nerves or systemic application of capsaicin were studied. A dense network of SP-IR nerves was observed in the right atrium in different locations: (1) around local cardiac ganglion cells, (2) close to blood vessels, (3) within the myocardium, and (4) close to and within peri- and endocardium. A moderately dense SP-innervation, mainly related to blood vessels, was found in the ventricles. Very dense networks of NPY- and TH-IR nerve fibers with an overlapping distributional pattern around blood vessels and in the myocardium were seen in both the atria and the ventricles. In addition, some cell bodies in local cardiac ganglia were NPY-IR. Bilateral stellatectomy resulted in a reduction of SP-IR in the right atrium (55% of control), which was more pronounced after additional capsaicin pretreatment of the vagal nerves (44% of control). In the left ventricle no significant depletion of SP-IR was seen by either stellatectomy or combined stellatectomy and capsaicin treatment of the vagal nerves. It was not possible to establish any defined target areas within the heart for vagal or spinal SP-IR afferents by use of immunohistochemical methods. Systemic capsaicin treatment caused a total loss of SP-IR nerves in the heart.(ABSTRACT TRUNCATED AT 250 WORDS)