Using a new ultrasensitive method the trace biogenic amines, phenylethylamine, meta-tyramine and para-tyramine have been quantitated in brain regions obtained from a single rat. Phenylethylamine concentrations in ng/g wet tissue (mean +/- std. error) were as follows: caudate 2.71 +/- 0.73, hypothalamus 0.45 +/- 0.15, cerebellum 0.09 +/- 0.02, olfactory bulb 0.35 +/- 0.11, stem 0.13 +/- 0.03, hippocampus 0.20 +/- 0.11, cortex 0.69 +/- 0.13 and the rest (remainder of the brain) 2.81 +/- 0.41. Mean whole brain was 1.23 +/- 0.19 ng/g, in agreement with previous measurements. meta-Tyramine concentrations (ng/g) were: caudate 2.69 +/- 0.19, hypothalamus 0.32 +/- 0.16, cerebellum 0.07 +/- 0.04, olfactory bulb 0.09 +/- 0.04, stem 0.04 +/- 0.01, hippocampus 0.07 +/- 0.02, cortex 0.18 +/- 0.15 and the rest 0.15 +/- 0.06, with a mean whole brain value of 0.26 +/- 0.05 ng/g and para-tyramine concentrations were: caudate 8.99 +/- 1.60, hypothalamus 0.93 +/- 0.13, cerebellum 0.78 +/- 0.27, olfactory bulb 0.70 +/- 0.13, stem 0.90 +/- 0.36, hippocampus 0.40 +/- 0.06, cortex 1.78 +/- 0.28 and the rest 2.38 +/- 0.12 and mean whole brain was 1.90 +/- 0.25 ng/g. In human plasma the concentrations of the three amines were found to be 31.3 +/- 3.4 pg/ml, 5.3 +/- 1.6 pg/ml and 66.0 +/- 9.9 pg/ml respectively and in dog blood 95.3 +/- 4.6 pg/ml, 24.0 +/- 7.6 pg/ml and 486 +/- 43 pg/ml respectively. When monoamine oxidase inhibitors were added to the blood immediately after collection there were no significant increases in the amine levels indicating that MAO-B is not present in plasma in significant quantities.