The vast majority of excitatory synaptic inputs to neocortical pyramidal cells terminate on dendritic spines, which can thus serve as markers, visible by light microscopy, for the locations of these synapses. The aim of this study was to provide estimates of the total numbers and distributions of spines on the dendrites of individual pyramidal neurones from layers 2/3 and 5 of the visual cortex of the rat. High magnification camera lucida drawings were made of dendritic segments lying close to the plane of section and the number of spines per unit length of dendrite calculated for each. These spine densities were used to estimate the numbers of spines on the other dendritic segments and the results were entered to a computer program that calculated various statistics. Mean total numbers of spines per cell were 7,965 +/- 2,723 (S.D.) for layer 2/3 cells, 8,647 +/- 3,097 for slender layer 5 cells, and 14,932 +/- 3,371 for thick layer 5 cells; these figures are in good agreement with previous stereological estimates. For all cell classes, 70% or more of spines were located on the basal and apical oblique dendrites. The distribution of spines with respect to cortical layers was also explored. Most cells had most of their spines in the layer containing the soma, but there were differences within and between cell classes. Layer 2/3 cells showed a progressive reduction in the proportion of their spines in layers 1 and 2 with increasing depth of their soma in the cortex. Thick layer 5 cells had substantial contributions from layers 4, 3, 2, and especially layer 1. Slender layer 5 cells had small contributions from layers 6 and 4, but relatively few spines in layers 3 and 2. The distribution of spines with path distance from the soma was explored by estimating the numbers of spines contained within a series of concentric shells centred on the soma. All cells showed a rapid increase in the number of spines per shell for the proximal 100 micrograms or so, followed by a sharp decline to approximately 250 micrograms, beyond which the number remained relatively constant until the end of the terminal arbor. In each case, the majority of spines were located within a path length of 150 micrograms from the soma.