We have previously documented a direct relationship between protein intake and renal excretion of inorganic sulfate (iSO4) and Ca in the newborn piglet. In humans these relationships are hypothesized to contribute to osteopenia of prematurity and adult osteoporosis. There is little information about urinary iSO4 in the newborn, which originates from sulfur amino acid metabolism. In this study we compared renal excretion of iSO4 to that of Ca and PO4. hypothesizing renal handling of iSO4 would parallel thai of PO4. 6 piglets (weight 1401±79g) were placed in a metabolic unit at 24 hours of age with unlimited access to a liquid diet (4.8 g/dl bovine protein, 126 mg/dl of Ca. 96 mg/dl of P) beginning at 48 hrs. of life. Formula intakes ranged from 533±124 on day 2 to 856±278 on day 8. protein intakes averaging 24.9±5.8 g/day and 40.1±13 g/day respectively. Ca intakes increased from 671±157 mg/day to 1079±350 mg/day and P intake from 511 mg/day to 821 mg/day. GFR (estimated from creatinine clearance) ranged from 1.96±0.36 ml/min on day 1 to 2.49±0.63 ml/min on day 8. Plasma iSO4 concentration increased from 23.93±3.45 mM/ml day 1 to 30.6±1.91 mM/ml day 8. Renal iSO4 excretion correlated directly with protein intake (r=.98) ranging from 50.62±17.89 mM on day 1 to 525±105 mM on day 8. Fractional excretion of iSO4 (FeiSO4) ranged Irom 0.08±0.03 on day 1 to 0.54±0.19 on day 8. Total PO4 excretion (33.8±27.6 mg day 1. 192±73.0 mg day 8) and FePO4 (0.11±.04 day 1, 0.62±0.15 day 7) also increased with increasing PO4 intake (r=0.70). This was in contrast to the minimal increases in urinary total Ca excretion (10.16±2.34 mg day 1, 34.54±18.14 mg day 8) and FeCa (0.04±.00 day 1, 0.07±.02 day 8) which did not correlate with increasing Ca intake (r=0.10) and increasing GFR. We conclude that renal excretion of PO4 and iSO4 (total and fractional) increases with increasing intake and GFR in the newborn pig. This is in contrast to Ca excretion which increases minimally in the newborn despite increasing GFR and Ca intake.