In this study, the detailed distribution of peptidergic nerve fibers in the sinus hair follicle was immunohistochemically investigated by the avidin biotin-complex method using antibodies against calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY) in the cat, dog, hamster, rat, mouse and guinea pig. The peptide-immunoreactive (IR) nerve fibers presented a fine varicose appearance. They entered the sinus hair follicle after penetrating the capsule (Cap) at various levels. Peptide-IR fibers in the sinus hair were classified into the 3 types described below based on their course of entry and terminal distribution. Type A: these fibers arrive at the orifice of the sinus hair follicle from superficial dermis and innervate the upper portion of the follicle. Most of them form a network around the vibrissal shaft (VS) in the outer and inner conical body (OCB and ICB, respectively), and some are distributed around the rete ridge collar (RRC) and the sebaceous gland (SG). Type B: these fibers enter the lower third of the sinus hair follicle after forming a nerve bundle together with myelinated fibers, or accompany an artery. After distribution in the trabeculae (Trab) of the cavernous sinus (CS), they form a dense plexus in the connective tissue follicle (CTF) at the level of the CS. Some ascend through the CTF and terminate at the level of the ring sinus (RS). Type C: these fibers enter the sinus hair follicle at its base. They innervate the hair papilla (HP) and the CTF of the hair bulb. CGRP and SP were detected in all types of nerve fibers in all species investigated. These peptidergic nerve fibers showed the same distribution pattern, but CGRP fibers were more numerous than SP fibers. They were distributed at high density in the OCB, ICB, CTF, ringwulst (Rw), the Trab of the CS and the HP. A moderate number of VIP- and NPY-IR fibers, mainly types B and C, were detected in these portions in the cat, dog and hamster, but few fibers were observed in other portions or in other species. Although the basic structures of the sinus hair follicle presented almost the same features as in the mammalian species, the connective tissue of the HP extended to the level of the RS in the cat, rat, hamster, and guinea pig. In these species, CGRP-, SP- and/or VIP-IR fibers extended to the top of the papillary connective tissue, and in the cat, were especially well developed.(ABSTRACT TRUNCATED AT 400 WORDS)