The osmolalities of pediatric i.v. admixtures were measured to identify drug concentrations in selected vehicles that would conserve fluid while maintaining osmolality values of 400 mOsm/kg or less. Test solutions were prepared by diluting appropriate volumes of freshly reconstituted powdered drug products or commercially diluted drug products with 5% dextrose injection, 0.9% sodium chloride injection, or both to provide 5 mL of each admixture at desired drug concentrations. To reduce their osmolalities, trimethoprim-sulfamethoxazole and ampicillin sodium were also diluted in 0.45% sodium chloride injection; ticarcillin disodium was diluted only in 0.45% sodium chloride injection. A vapor pressure osmometer was used to measure osmolalities in triplicate for three solutions prepared for each admixture. Of the 63 different admixtures prepared with 5% dextrose injection or 0.9% sodium chloride injection or both, 47 (75%) had osmolalities of 400 mOsm/kg or less. At least one concentration of each selected drug diluted in these vehicles had an osmolality of less than 425 mOsm/kg, except for trimethoprim-sulfamethoxazole and ampicillin sodium. Selected concentrations of the latter two drugs and ticarcillin disodium in 0.45% sodium chloride injection resulted in acceptable osmolalities. For most drugs diluted to the same concentration in 5% dextrose injection and 0.9% sodium chloride injection, osmolalities were lower in the dextrose solutions. Selection of an appropriate vehicle and drug concentration can control the osmolality of i.v. admixtures when the volume of fluid must be minimized, as for pediatric patients.