The geometry of the arteriolar network is one of the major determinants of blood flow distribution within a tissue. The purpose of this study was to describe the distribution of geometrical variables (lengths, diameters) as well as the pattern of branching in the nonarcading portion of the arteriolar network in skeletal muscle. The exteriorized cat sartorius muscle was used as the experimental model. The intravascular fluorescence of fluorescein isothiocyanate (FITC)-labeled Dextran 150 was observed with a low-light-level video camera, and the vascular networks were mapped. Arteriolar lengths and diameters were measured, and vessel position in the network was characterized by Strahler's method of ordering, in which the first-order arterioles give rise to most of the capillaries. Typically, the nonarcading, terminal networks contain three or four arteriolar orders. The sequences of the number of vessels, mean diameter, and mean length for each order are accurately described by geometric progressions (Horton's law). The distribution of diameters within each order was rather narrow: typically two-thirds of the vessels fell within 20% of the mean value. The spread was reduced by half when vessels within a single network were considered. During vasodilation to a standard stimulus the relative dispersion of diameters increased modestly. The distribution of vessel lengths was broader than for diameters. Two-thirds of vessels of a single order fell within 50-75% of the mean. The spread was less within individual networks. The variability of vessel geometry and branching patterns was substantially less within a single network than for a population drawn from a group of networks.