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The influence of infusion of hydralazine, isoxsuprine or nylidrin on blood flow in the basilar artery and on systemic blood pressure was investigated in anesthetized dogs. The drugs were administered individually in 1-mg and 2-mg doses and in various combinations of 1 mg each by injection just proximal to the formation of the basilar artery. Analysis of the data revealed significant increases in blood flow (P<0.05) immediately after injection and a statistically significant (P<0.05) interaction between time and drug. No statistical differences were found in comparisons of the effects of the 1-mg and 2-mg dose levels for any of the drugs. At either dosage isoxsuprine and nylidrin caused a 16% to 28% decrease in systemic blood pressure, while hydralazine effected a more moderate, approximately 5%, decrease. Basilar flow responded to 1-mg combinations of isoxsuprine/hydralazine and nylidrin/hydralazine with a consistent 25% to 30% increase. However, the flow response to nylidrin/isoxsuprine was unpredictable. The data indicate that these substances increased basilar arterial flow by inducing a transient local vasodilatation. The effects of the drug combinations on flow were in general synergistic and positive, and were similar to those after infusion of the individual drugs, except for a somewhat slower initial response and a longer duration.