Instrumented dogs were presented with two daily avoidance conditioning sessions during 12 days of continuous'saline infusion (1.3 I/day). Avoidance conditioning of sodium-loaded dogs resulted in progressive 24-h hypertension over the 12-day conditioning periods (systolic, 21 ±3 mmHg; diastolic, 15 ±1 mmHg) accompanied by a decreased 24-h heart rate (-14.8 ± 4.0 beats/min). Under these conditions, renal excretion of sodium decreased relative to sodium intake (-88 ± 19 mmol/12days) while urine volume was increased relative to water intake (0.18 ± 0.07 I/day). The sodium retention was accompanied by increased plasma sodium levels (1.8 ± 0.7 mmol/l) and decreased plasma calcium levels (-1.2 ± 0.2 mmol/l). Daily creatinine clearance decreased during the development of hypertension (-53 ± 13% per day). Subsequently, each dog was exposed to 12 days of saline infusion in the absence of avoidance sessions. Under these conditions, arterial pressure and sodium balance remained stable. It was concluded that the rapidly developing and reversible hypertension occurring in sodium-loaded dogs exposed to recurrent behavioural stress is mediated by increased levels of total body sodium.