This study was designed to determine the effects of a rapid bolus dose of atracurium 0.6 mg kg-1 on arterial pressure, heart rate and plasma histamine concentration (n = 9), and to compare these values with those obtained by (a) giving the same dose of atracurium slowly (over 75 s) (n = 9), or (b) pre-treating with H1- and H2- antagonists (n = 9). The rapid (5-s) bolus dose of atracurium i.v. resulted in a significant increase in plasma histamine concentration (P less than 0.05) and was associated with a decrease in mean arterial pressure and an increase in heart rate. Administering the same dose of atracurium slowly (over 75 s) prevented the increase in plasma histamine concentration, and abolished the subsequent haemodynamic response. Pretreatment with cimetidine 4 mg kg-1 i.v. and chlorpheniramine 0.1 mg kg-1 i.v. abolished the haemodynamic response despite a moderate increase in histamine concentration (0.1 greater than P greater than 0.05).