The relationship between the changes in plasma vasopressin (AVP) concentration and urinary concentration during left atrial distension has been examined in 12 anaesthetized dogs. Left atrial pressure was increased by 1.2 kPa for 30 min. Plasma AVP concentration (radioimmunoassay) was decreased 5 min after the start of atrial distension and was increased again 5 min after the end of distension. The average decrease was about 50% from a mean of 6.4 +/- 2.4 pg X ml-1 (SE). Urine osmolality decreased more slowly reaching its lowest value in the first 10 min after removal of atrial distension. In contrast sodium excretion increased immediately upon atrial distension. Because of the difference in the time course of the changes in plasma AVP and urine osmolality, plasma AVP was compared with the urine osmolality in samples collected 15 min after the plasma samples. At any plasma AVP concentration there was a wide variation in urine osmolality between dogs, but in any one dog there was clear relationship between plasma AVP and urine osmolality. The results support the view that the diuretic response to left atrial distension is due, at least in part to decreases in plasma AVP concentration. They also show that a stimulus arising from increased left atrial pressure influences the relationship between plasma osmolality and plasma AVP concentration.