The changes in metabolic rate (MR) and plasma noradrenaline (NA) were studied in rats injected with NA either subcutaneously (S.C.) or intravenously (I.V.). Injection of NA (400 micrograms kg-1 S.C.) raised the MR by 54.0 +/- 10.2% above baseline and the levels of plasma NA were higher in right atrial venous plasma (147.0 +/- 6.6 nmol l-1) than in arterial plasma (54.4 +/- 6.6 nmol l-1) 90 min after the injection. There was no significant increase in MR in response to the infusion of 0.4 microgram NA min-1 kg-1 but significant responses occurred (P less than 0.01) with infusion of 1.5, 3.0, 4.0 and 8.0 micrograms min-1 kg-1. The maximum increase in MR was 90.3 +/- 7.4% during the 3.0 micrograms min-1 kg-1 infusion. The arterial plasma NA levels were directly proportional to the NA infusion rate, 18.0 +/- 8.3, 72.4 +/- 14.3, 159.5 +/- 28.4, 222.3 +/- 62.1 and 590.0 +/- 172.1 nmol l-1 for the 0.4, 1.5, 3.0, 4.0 and 8.0 micrograms min-1 kg-1 infusion respectively. This study shows that high plasma levels of NA are needed to induce thermogenesis when NA is administered either S.C. or I.V.