The effects of local anesthetics (ketamine HCl, lidocaine HCl, procaine HCl, and tetracaine HCl) on stage 8 (four-somite) chick embryos were investigated. In general, embryos responded to drug treatment in a dose-related manner during the first 6 hr of incubation. Concentrations of 500 micrograms/ml (ca. 2 mM) or higher were embryolethal, whereas 100-200 micrograms/ml (0.1-0.8 mM) preferentially inhibited elevation of neural folds. The latter effect was detectable within 3 hr of treatment and was readily reversible. Tetracaine was the most potent among the four local anesthetics tested at any given dose. Compared to controls, cells in the defective neuroepithelium were less elongated and exhibited smoother apical (luminal) surfaces, thinner microfilament bundles, and less intense actin-specific fluorescence. Furthermore, the effects of local anesthetics (100-200 micrograms/ml) on stage 8 chick embryos were not identical to those of cytochalasin D (0.05 micrograms/ml), colchicine (1 microgram/ml), or ionophore A23187 (25 micrograms/ml), although all treatments produced neural tube defects. Overall results suggest that local anesthetics inhibit closure of the neural tube through their disruptive action on the organization and function of microfilaments in developing neuroepithelial cells.