The granule cell dendritic fields of the adult rat dentate gyrus were analyzed quantitatively using a probabilistic method developed to correct dendritic length and segment number for dendrites cut during sectioning. Golgi-impregnated, linearized hippocampi were sectioned serially in one of the three hippocampal planes. Three dendritic field parameters were quantified from camera lucida drawings of these dendritic fields: dendritic field spread, dendritic length, and the branching and termination patterns of dendritic segments. Granule cell dendritic fields resembled cones, their maximal extent occurring in the distal third of the molecular layer. The ratio of transverse to longitudinal dendritic field spread was greater than 1:1 for the dorsal leaf and crest regions, but close to or less than 1:1 for the ventral leaf. The probabilities of segment branching and termination were highly similar for transversely and longitudinally sectioned tissue. The probability of branching varied among dendritic orders and across the molecular layer for the same order. The probability of termination did not vary greatly across orders. Most nonbranching segments terminated adjacent to the hippocampal fissure. On the average, a granule cell had 2.23 first-order dendrites that branched into a dendritic field containing seventh-order dendrites. Total dendritic length, corrected for cut dendrites and projection errors, averaged 3,662 +/- 88 microns. The somatic layer and proximal third of the molecular layer contained approximately 35% of this total length. The remainder, ca. 60%, was restricted to the distal two-thirds of the molecular layer, the predominant termination zone of perforant path axons. These data provide a quantitative characterization of the rat granule cell dendritic fields. Implementation of the probabilistic correction method overcomes methodological problems common to quantitative Golgi studies. These data permit a more precise relationship to be drawn between dendritic architecture and granule cell physiology.