Plasma dopamine-beta-hydroxylase (DBH) activity is a controversial index of sympathoadrenal function. In our results, the half-life of bovine DBH administered by cardiac puncture to Wistar rats was dependent on plasma glucose values, being 60 min for controls, 96 min for streptozotocin (STZ)-diabetic animals (p less than 0.02) and 33 min for insulin-treated normal rats (p less than 0.01). In experimental situations with low plasma glucose levels, DBH activity was also diminished with respect to controls (glucose: 103.6 +/- 2.2 mg%, DBH: 9.7 +/- 0.5 U/ml). After fasting, glucose was 60.8 +/- 1.5 mg% (p less than 0.001) and plasma DBH 6.4 +/- 0.3 U/ml (p less than 0.001); fasting plus cold exposure also decreased glucose (66.2 +/- 1.4 mg%, p less than 0.001) and plasma DBH (6.7 +/- 0.2 U/ml; p less than 0.001). In both situations, there was an increase in exocytosis from sympathoadrenal tissues; however, no increase in plasma DBH levels was observed, because plasma glucose being diminished it was unable to compete at the catabolic receptor level. When normal plasma glucose levels take place, plasma DBH is essentially constant, poorly reflecting a moderate increase or decrease in exocytosis from tissues, as was the case in our animals with 48 h of cold exposure. When chemical sympathectomy (6-OH-dopamine) or bilateral adrenalectomy was performed there was a compensatory mechanism between them. Plasma DBH does not change significantly in these situations if plasma glucose values are normal. From these results, the most important physiological influence on plasma DBH activity is the glucose plasma levels. Plasma DBH values not being a useful index of sympathoadrenal activity if, at the same time, the plasma glucose levels are not considered.