The effects of prenatal exposure to ethanol on the structure and function of corticospinal neurons was investigated. The subjects were the 3-4-month-old offspring of hooded rats fed a nutritionally balanced liquid diet containing 6.7% (v/v) ethanol (Et), pair-fed a nutritionally matched isocaloric diet (Ct), or fed chow and water (Ch). Corticospinal neurons in primary somatosensory cortex were examined by intracellularly recording and filling cells that were driven by antidromic stimulation of the pyramidal decussation. In the control rats, corticospinal neurons comprised a homogeneous morphophysiological population. Morphologically, all of the antidromically driven cells examined were pyramidal neurons with cell bodies in layer Vb. The dendrites of these neurons were spinous and branched within layers I, IV, and V. Their axons arborized within layers IV, V, and VI and some collaterals extended laterally for distances up to 2.6 mm from the cell body. The mean conduction latency was 3.6 and 3.4 msec for Ch- and Ct-treated rats, respectively. In Et-treated rats, corticospinal neurons constituted a heterogeneous population. The laminar distribution of the corticospinal neurons in Et-treated rats was broad; the cell bodies of labeled neurons were in layers II, IV, V, and VI. The dendrites of layer Vb neurons were spinous; however, many of the spines appeared dysmorphic and the density of spines was significantly greater (32%) in Et-treated rats than in Ct-treated rats. Although the dendritic branching pattern for layer Vb neurons was similar to that described for the controls, a Sholl analysis showed that the complexity and extent of their dendritic trees were significantly greater in Et-treated rats. The axons of all layer Vb neurons in Et-treated rats had long horizontal processes that arborized in layers IV-VI, and some neurons also had an array of collaterals that ascended to layer I. The mean conduction latency for layer Vb neurons was 3.9 msec. The structure and function of ectopic neurons (those in layers II, IV, Va, Vc, and VI) in Et-treated rats differed markedly from those of the layer Vb neurons. Morphologically, the dendritic and axonal fields of these neurons were narrower than for the layer Vb neurons, and the ectopic neurons had a mean conduction latency of 7.1 msec. The heterogeneity of the population of corticospinal neurons in Et-treated rats may result from the effects of ethanol on early events in neuronal development such as neuronal generation and migration.