Correlation between hemodynamic changes and plasma catecholamines or renin activity were studied in dogs anesthetized with pentobarbital, 30 mg/kg, i.v. Pentobarbital increased heart rate in all cases regardless of changes in plasma catecholamines, and the increase was not depressed fully by propranolol. Blood pressure (BP) showed a transient decline just after pentobarbital injection and then elevated gradually to one of three levels 30-60 min after anesthesia. In low (62 +/- 4 mmHg, Mean +/- S.E.) and very high BP (169 +/- 7 mmHg), plasma norepinephrine (NE) showed low (305 +/- 55 leads to 89 +/- 22 pg/ml) and high levels (296 +/- 54 leads to 372 +/- 106 pg/ml), respectively, which probably reflect the neurosympathetic activities. In moderately high BP (112 +/- 4 mmHg), however, the hypertension did not necessarily reflect changes in plasma NE. When the level before anesthesia was 108-164 pg/ml, plasma NE increased, whereas it decreased when the level was 182-374 pg/ml. In either case, the level was fixed within 117-182 (154 +/- 6) pg/ml. Plasma renin activity increased after anesthesia and maintained for at least 2 hr. However, the increase were observed regardless of the BP level, and moderately high BP was not depressed by an angiotensin II antagonist. Participation of angiotensin in these sustained BP cases seems unlikely. Plasma epinephrine reflecting adrenal-medullary activity was decreased markedly in all cases, and the low level lasted for at least 2 hr. Changes in plasma dopamine are related to those in plasma NE, and the origin seems to be the same.