Recently, it was found in studies in vitro and in vivo that phencyclidine hydrochloride (PCP, 'angel dust') can induce cerebral arterial and arteriolar spasms, in psychotomimetic concentrations, by acting on specific PCP receptors, which is followed by rupture of cerebral and postcapillary venules. We wondered whether a chemical substance which has the ability to block Ca2+ channels, neurotransmitter release, intracellular Ca2+ release and the NMDA-glutamate receptor channel, viz., Mg2+, might block the PCP receptor which subserves cerebral contractile events and thereby prevent rupture of microvessels. In vivo experiments carried out on cerebral microvessels in a rat pial brain preparation revealed that different dose regimens of Mg aspartate HCl administered intravenously attenuated cerebrovasospasms induced by PCP and shifted PCP concentration-effect curves (ED50) rightward to higher concentrations. These data suggest that Mg2+ may alter the binding of PCP for its vascular receptors. Since Mg2+ prevented rupture of the cerebral microvessels, it may prove useful, clinically, in prevention and treatment of PCP-intoxicated victims.