The in vivo release rates of endogenous noradrenaline from the hypothalamus and dopamine from the caudate nucleus of the rat have been determined. Artificial CSF perfusates collected from a push-pull cannula inserted into specific areas of the brain were assayed for the amines by a sensitive radioenzymatic procedure. The release rates of noradrenaline and dopamine into artificial CSF perfusates were 38 +/- 6 and 46 +/- 6 pg/h (225 +/- 36 and 301 +/- 39 fmol/h), respectively; when 0.5 mM amphetamine was added to the CSF, the release rates of noradrenaline and dopamine increased to 176 +/- 50 and 1183 +/- 453 ph/h (1041 +/- 296 and 7732 +/- 2961 fmol/h), respectively.