Primary cultures of liver cells isolated from seven rats were used to study the possible toxicity of atracurium and metocurine. The muscle relaxants were separately added to the culture medium and the cells then incubated for 4 hr. The amount of lactic dehydrogenase (LDH) that leaked into the culture medium was determined at the end of incubation. The customary assumption was made that the exudation of LDH reflects the toxic effects of the relaxants. In untreated dishes, approximately 11% of the total intracellular LDH leaked out during the incubation. The net leakage of LDH produced by the relaxants was obtained by subtracting this amount from the LDH activity determined in the media of dishes with the relaxants added. On this basis, metocurine, in concentrations of 12-850 X 10(-6)M, did not cause a net leak of LDH. On the other hand, atracurium, in similar molar concentrations, caused a statistically significant and concentration-dependent leak of LDH that, at its maximum, amounted to more than one half of the intracellular LDH. The results are interpreted in terms of damage to cellular membranes produced by atracurium or its metabolites. Although the exact biochemical process was not identified, we hypothesize that acrylates--produced by Hofmann elimination from atracurium--might be the likely toxic species.