The effect of essential fatty acid deficiency (EFA-D) on the plasma concentration of arginine-vasopressin (AVP) and the urinary AVP excretion was investigated. Weanling rats were fed a fat-free diet (FF-rats). Control rats received the same diet in which 6% by wt. of sucrose was replaced by arachis oil. After 4-6 weeks of feeding, urine and plasma were analysed for AVP, osmolality, sodium and potassium. When compared to control rats FF-rats had decreased urine volume (6.0 ± 1.6 ml/24 hr versus 11.7 ± 3.2 ml/24 hr), increased urine osmolality (2409 ± 691 mOsm/kg versus 1260 ± 434 mOsm/kg), increased urinary AVP excretion (5.52 ± 1.62 ng/24 hr versus 2.19 ± 1.20 ng/hr and increased plasma AVP (2.55 ± 0.45 pg/ml versus 1.95 ± 0.25 pg/ml). There was no difference between the groups in plasma osmolality. The triene/tetraene ration was 0.78 + 0.15 (n = 12) in total kidney lipids of the FF-rats. The triene/tetraene ratio was positively correlated with urinary AVP excretion. It is concluded that FF-rats had elevated plasma AVP concentration and disturbed water balance, both of which probably were provoked by an increased transepidermal water loss.