Factors affecting calcium-phosphate solubility in parenteral nutrition solutions used in neonates were studied. Six neonatal parenteral nutrition solutions were prepared using either Aminosyn or FreAmine III and various amino acid and dextrose concentrations. Phosphorus (as mono- and dibasic potassium phosphate) and calcium (as 10% calcium gluconate) were added in concentrations of calcium 2.5-100 meq/liter and phosphorus 2.5-100 mmol/liter. Duplicate samples were prepared and analyzed either after they were heated in a water bath (37 degrees C) for 20 minutes or after 18 hours at 25 degrees C followed by 30 minutes in a water bath (37 degrees C). Precipitation was detected visually and spectrophotometrically, and pH was measured. Lipid emulsion was added to two Fre-Amine III solutions in a ratio of 7.5:1 (parenteral nutrition solution:lipid) and the resulting pH was measured. Time and temperature affected calcium-phosphate solubility in all solutions tested. Precipitation curves of amount of calcium versus amount of phosphate added were prepared for each solution. Amino acid and dextrose concentrations affected the pH of the solutions, and when a lipid emulsion was added, the pH rose more in the 1% than in the 2% FreAmine III solution. In selected solutions, as much as 120 mg/kg/day calcium and 55 mg/kg/day phosphate can be administered, approximating daily third-trimester accumulation of these minerals. Use of the precipitation curves in this paper, with attention to their limitations, should aid in the safe delivery of calcium and phosphorus intravenously to neonates.