The administration of atracurium to humans has been reported to produce little cardiovascular effects in clinical doses. The cardiovascular effect, histamine-releasing and catecholamine releasing effects of intravenous injection of atracurium 0.6 and 1.2 mg.kg(-1) were studied in man, and also the cardiovascular and catecholamine releasing effects of laudanosine which is a metabolite of atracurium by Hofmann degeneration, were studied in dogs. The increase in human plasma concentration of histamine, hypotension and tachycardia were found with the dose of atracurium 1.2 mg.kg(-1). The intravenous administration of laudanosine 10 micro g.kg(-1) to dogs produced minimal epinephrine, norepinephrine releases and cardiovascular changes.