The cutaneous microvasculature consists of an upper horizontal plexus in the papillary dermis from which the capillary loops in the dermal papillae arise, and a lower horizontal plexus that lies at the interface of the dermis and fat. Connecting the upper and lower plexuses are direct arborizing channels from which the capillary networks around the hair follicles and sweat glands arise. The predominant vessels in the upper plexus are postcapillary venules 10-20 mum in diameter which are the sites of (I) inflammatory cell emigration, (2) vascular permeability produced by histamine, and (3) deposition of immune complexes in vasculitis. The lower plexus consists of arterioles and venules 50 mum in diameter which have a different ultrastructure from their counterparts in the upper plexus. The dermal lymphatic network constantly remodels itself in response to inflammation and epidermal hyperplasia. Although physiological studies traditionally treat the dermal microcirculation as a homogeneous unit, attempts should be made to relate specific physiological function with microvascular ultrastructure.