Catecholamines have multiple metabolic and fluid-electrolyte as well as cardiovascular effects, and their levels in plasma respond to alterations in nutrient and sodium intakes. Plasma norepinephrine, epinephrine, and dopamine were measured in 12 obese nondiabetic subjects before and after 400 kcal/day diets of either protein or glucose, followed by total fasting, and then by hypocaloric refeeding, each for 14-day periods. Measurements were made in the supine and upright posture and during and after 6-10 min of exhaustive exercise at 80% maximal VO2. Sodium intake varied with the nutrients ingested, being markedly decreased with the 400 kcal/day diets and fasting. Norepinephrine levels were higher in fasting than base line while subjects were lying or standing and after recovery from exercise. Those of epinephrine were elevated on standing. No differences were found after the 400 kcal/day diets, with protein and glucose yielding equivalent results. In contrast, refeeding was associated with lower norepinephrine levels than all other diets while subjects were lying or standing and after recovery from exercise. Peak levels of norepinephrine and epinephrine with exercise were indistinguishable among diets. Fasting was associated with lower systolic and diastolic blood pressures and lower responses to standing and exercise than base line, whereas upright heart rate was greater and that during exercise less than base line. Significant though less extensive changes occurred with the other diets. Thus both electrolyte status and nutrient intakes interact to determine net catecholamine responses, and the former seem to override the latter.(ABSTRACT TRUNCATED AT 250 WORDS)