In order to quantify losses of Na and K associated with precipitate fractions in semi-solid avian urines, Na, K, uric acid and urates (UA + U) and inulin were measured in plasma, in whole ureteral urine, and in urinary precipitates of hens. Ten animals were used, five fed a control commercial poultry meal (Diet A) and five maintained on a high protein, low-Na feed (Diet B). 2. In ureteral urines from hens on Diet A, dried precipitate accounted for 5.6% of the total whole urine weight, on the average. UA + U constituted about two-thirds of the precipitates' weight and 80% of the total excreted UA + U load of 97.4 mM. The average molar fractions, [Na]/[UA + U] and [K]/[UA + U], in precipitates were in the range 0.1-0.2; and the Na and K lost in these fractions were 12.8 mequiv and 9.6 mequiv, respectively, per 1 whole urine. These losses represent 9% and 23% of total Na and K excretion. 3. Diet B was used to accentuate potential cation loss in precipitates if obligatory binding of K and especially Na were to occur in precipitates. Urinary [UA + U] in whole urine rose to 146 mM of which 95% was found in precipitates. The average molar fraction [Na]/[UA + U], however, fell to 0.06 and that of [K]/[UA + U] to 0.08. Renal loss of Na was 8.5 mequiv and of K was 11.5 mequiv per 1 whole urine. 4. These experiments reveal that significant but minor fractions of excreted Na and K are associated with precipitates of avian urine, although the loss is insignificant compared to that reported in starlings (Braun, 1978). They further indicate that the Na/inulin clearance ratio, based on measurements in whole urine and plasma, adequately reflects fractional excretion which, on Diet A, was 1.8% of the filtered load and, on the Na-poor Diet B, less than 0.1%. These values place the Na-reabsorbing abilities of these birds easily within the range reported for ureotelic vertebrates and suggest that uricotelism does not impose a major renal salt loss in birds.
