Six women were given 1 g of piperacillin on the 2nd postpartum day and serum concentrations of the drug were measured at 0.5, 1, 2, and 4 h after infusion. The same women returned for an identical test 6 months later and hence served as their own controls. A second group of 6 women were treated identically except that they received the first dose of piperacillin on the 3rd postpartum day. The serum levels of antibiotic were consistently lower during the puerperium than at 6 months. The levels obtained at 2 compared to 3 days post partum were similar, but were about one half the levels obtained at 6 months post partum. These results indicate that antibiotic therapy for postpartum endomyometritis may need to take account of the fact that the altered pharmacokinetics of pregnancy persists into the puerperium. If used in this setting, the dose of piperacillin may need to be increased by 50% over the normal adult dose.