Concentrations of ibuprofen in breast milk and serum were compared in 12 patients who had ingested one 400 mg tablet of ibuprofen every 6 hours over a 24-hour period for relief of post-cesarean section pain. Samples of breast milk and blood were obtained simultaneously over a 34-hour period beginning just prior to the first dose of ibuprofen. Gas-liquid chromatography assay methodology capable of detecting 1 microgram/ml was used to determine concentrations of ibuprofen in serum and breast milk. Ibuprofen was present in the serum with a half-life of approximately 1.5 hours. No measurable amounts of ibuprofen were found in the samples of breast milk. The conclusion drawn is that, in lactating women who take up to 400 mg of ibuprofen every 6 hours, less than 1 mg of ibuprofen per day is excreted in breast milk.