The extrauterine development of marsupial pouch young (northern brown bandicoot Isoodon macrourus) has facilitated the study of the effects of murine epidermal growth factor (mEGF) on pattern formation in skin. Hair follicle initiation and development, which in the mouse would occur from about Days 13-14 of gestation onward, occurs postnatally. In the present study the effect in vivo of mEGF on developing skin corresponding to mouse gestational ages from Day 13 onward was examined. Subcutaneous injections of mEGF (0.5, 1.0 and 2.0 microg g[-1] body weight) or equivalent volumes of saline (0.9% w/w) were administered daily, before and during hair follicle initiation and development. Murine EGF inhibited the formation of hair follicles, hair follicle sweat glands, sebaceous glands and dermal papillae. The pattern of follicle initiation was perturbed. The characteristic trio follicle grouping was absent, and follicle rudiment densities (no. per mm2 skin surface) were significantly lower in animals treated with mEGF, whereas follicle diameters were increased. These data may reflect a role for the epidermal growth factor (EGF) receptor in epidermal pattern formation. The EGF receptor and its potential ligands (such as EGF, transforming growth factor (TGF-alpha) or other yet-to-be-discovered ligands) perhaps act as parts of a pattern-forming system in vertebrate skin.