This study examines the consequences of sodium chloride supplementation to young rats previously made salt deficient by feeding them a sodium-deficient, chloride-replete diet. Salt-deficient rats received the test diet and distilled water for 10 days. As in our previous studies, rats cared for in this manner grew more slowly than rats fed the identical diet but allowed to drink 37 mM sodium chloride. On day 11, half of the salt-depleted animals received 37 mM sodium chloride in their drinking water. Sodium-deficient and supplemented rats were studied 1,2,5-6 and 11-12 days later. Urinary sodium rapidly rose from undetectable of 46 mEq/l urine within 1 day of supplementation and there was no further increase the next day, suggesting that extracellular fluid volumes were rapidly repleted. Food intake increased in the supplemented rats compared with the deficient animals but the difference in food intake equalled only 2.25 g/day for the first 2 days of supplementation. Over the last 12 days of the study, the slopes of both weight and length gains were equal in both the supplemented and the control group and significantly higher than those in the deficient rats. Over the course of the study, full catchup was not obtained in either length or weight. In addition to total weight and length gains, liver and kidney weights increased proportionately and by 5-6 days of supplementation were equivalent to the weights seen in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)