The hypothesis was tested in vitro that the spontaneous degradation of atracurium leads to formation of electrophilic metabolites. Variable amounts of atracurium were incubated in saline (0.9% NaCl) for 120 minutes at pH 8.0 and 370C. Subsequently, cysteine was added to the incubation solutions and the incubation was continued at pH 7.4 and 370C. Frequent determination of the mercapto groups of cysteine revealed a progressive diminution of the mercapto groups remaining in the incubation solutions. The consumption of sulfhydryl groups was maximal at 20 minutes after the addition of cysteine and amounted to approximately twice the molar amount of atracurium. Kinetic analysis indicated that one mercapto group was consumed almost instantly, whereas the consumption of the other proceeded with a half-life of 4 minutes. No consumption of mercapto groups was observed when laudanosine was incubated with cysteine. Incubation of atracurium or of its degradation products with carboxylesterase markedly reduced the amount of reactive metabolites present in the incubation solutions. The results are compatible with the working hypothesis that spontaneous degradation of atracurium via Hofmann elimination results in generation of two equivalents of reactive electrophilic esters, probably acrylates. We propose that in vivo the portion of the aliphatic chain in the atracurium molecule that is converted to acrylates by Hofmann elimination may be eliminated in part in urine as a conjugate of mercapturic acid.