A method for demineralization of bone, preserving the antigenicity of neuroactive peptides, was developed. In all parts of rat long bones, nerves immunoreactive to substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY) and tyrosine hydroxylase (TH) were detected after immunohistochemical staining. The majority of nerves were vascular, although several non-vascular endings were observed at the growth plate and amidst marrow cells. An abundance of nerves were demonstrated near the epiphyseal plate and in the periosteum, regions of high osteogenic activity. The occurrence of different nerve types was analyzed at different stages of heterotopic osteogenesis, induced by allogeneic bone matrix. Nerve fibres immunoreactire to SP, CGRP, N P Y and TH occurred amidst differentiating chondroblastic cells in the second week. They gradually increased in number during the ensuing eight weeks. In an in vitro study of osteoblastic cells (UMR 106-01, R O S 1 7/2.8, Saos-2, MC3T3-E1) receptors to CGRP, VIP, noradrenaline (NA) and N P Y were demonstrated as assessed by analysis of cyclic A M P formation. In UMR cells, N P Y inhibited the effects of NA and parathyroid hormone (PTH), which is the first demonstration of a receptor interaction between a local neuropeptide and a systemic calcium regulating hormone. The combined findings indicate a neuroendocrine influence on bone physiology.