The long-term objective of this project is to assess the effects of altering brain histamine on cardiovascular function in the conscious rat. Brain histamine will be raised by inhibition of histamine methyltransferase by means of SKF-91488 (homodimaprit). Brain histamine will be lowered by inhibition of histidine decarboxylase with Alpha-fluoromethylhistidine. We shall test the hypothesis that increasing histamine in selected brain areas raises blood pressure whereas decreasing it lowers blood pressure. In studying the mechanism for these effects, selective H1 and H2 agonists and antagonists will be injected into specific brain areas. These effects will be compared in the normotensive versus hypertensive animal.