The comparative central nervous system (CNS) toxicity of serially administered intravenous doses of lidocaine, bupivacaine, etidocaine, and tetracaine was investigated in awake dogs. The mean cumulative dose required for convulsive activity was 4.0 mg/kg tetracaine, 5.0 mg/kg bupivacaine, 8.0 mg/kg etidocaine, and 22.0 mg/kg lidocaine. The cumulative convulsive dose of lidocaine was significantly greater than that of the other three agents (P less than 0.01). A comparison of the in vivo anesthetic potency and the acute CNS toxicity of these various agents suggests little difference in the therapeutic ratio between less potent anesthetics such as lidocaine and more potent drugs, i.e., tetracaine, bupivacaine, and etidocaine. The relative CNS toxicity of the different agents as determined in awake dogs in this study was compared with their relative cardiovascular toxicity previously evaluated in a series of ventilated dogs anesthetized with pentobarbital. The dose of lidocaine, etidocaine, tetracaine, and bupivacaine required to produce irreversible cardiovascular depression was 3.5-6.7 times greater than that which produced convulsions. These results suggest that the CNS is the primary target organ for the toxic effects of both highly lipid-soluble and highly protein-bound local anesthetics (i.e., bupivacaine, etidocaine, and tetracaine) and less lipid-soluble and less protein-bound drugs (i.e., lidocaine) following rapid intravenous administration.