Hemodynamic effects of the kallikrein-kinin system can be investigated by experimental administration of specific kinin antagonists and by measurement of kinin levels in the circulating blood. In conscious normal rats, the bradykinin analog B4162 blunts the hypotensive effect of exogenous bradykinin. This kinin antagonist has no blood pressure effect in control rats, but it enhances the pressor effect of vasoconstrictor substances such as vasopressin or angiotensin II when they are infused at subpressor doses. Endogenous kinins may therefore participate in blood pressure regulation by antagonizing pressor substances. Plasma levels of endogenous kinins are normally in the low picomolar range. They are rapidly generated and destroyed in biological fluids. Thus, measurement of plasma kinins requires sensitive assays based on high-affinity antibodies and careful sample-handling techniques. Nonpolar solid-phase extraction on phenylsilylsilica provides a rapid, reliable, and easy extraction of kinins from plasma with constant and high recoveries.