Since the increase in intracranial pressure produced by succinylcholine is temporally associated with intravenous administration, we investigatedin vitroa possible direct cerebrovascular effect of this nicotinic drug. Isometric responses were recorded from dog and guinea pig basilar artery rings suspended in modified Krebs' solution at 37° C. After precontracting with a voltage (KCl)- or a receptor (5-hydroxytryptamine)-mediated agonist, cumulative concentration-relaxation curves were established for: pure succinylcholine; Quelicin® from multidose vials containing 20 mg/ml succinylcholine 1.8 mg/ml methylparaben, and 0.2 mg/ml propylparaben; Anectine® from single-dose vials containing 20 mg/ml succinylcholine; multidose Anectine® containing 20 mg/ml succinylcholine and 1.0 mg/ml methylparaben; and methylparaben and propylparaben alone. When required, the endothelium of dog artery was removed by gentle mechanical rubbing and the response to the drugs reevaluated. Both Quelicin® and multidose Anectine® produced statistically significant (P< 0.05) relaxation; Quelicin® was the more potent of the two. Methylparaben and propylparaben produced relaxation in an additive manner and completely accounted for the relaxation produced by Quelicin® and multidose Anectine®. The vascular relaxation was found to be independent of the presence of a functional endothelium. Consistent with a nicotinic induced contraction, pure succinylcholine maintained vessel tone. It is concluded that the pharmaceutically ubiquitous preservatives methylparaben and propylparaben but not pure succinylcholine have vasoactive propertiesin vitro.