Prostaglandin F2alpha constricted pial arterioles when locally applied to the cerebral surface. Norepinephrine and serotonin each elicted similar contractile effects. The constriction produced by F2alpha in combination with either biogenic amine was greater than the constriction elicited by F2alpha or amine acting alone. The effect of one agent on the other was additive rather than potentiating. Since F2alpha norepinephrine and serotonin are all naturally occurring agents, it is possible that their combined effect is important under pathological circumstances and this combined effect should not be overlooked in the search for single spasmogens of great potency. Before ascribing a pathologically important effect of F2alpha, either alone or in combination, evidence is required showing that doses effective in experiments are similar to the concentrations occurring during disease states and/or that vessels may become hypersensitive to F2alpha during such states.