The pharmacokinetics of cephradine, a cephalosporin with a low degree of protein binding, was studied in 12 women after oral and intravenous administration of the drug during and after pregnancy. Six of the 12 women also received a cephalosporin with a high degree of protein binding, cefazolin, intravenously during and after pregnancy. For both drugs most pharmacokinetic parameters were altered in pregnancy. The area under the plasma concentration-time curve (AUC) following intravenous administration was smaller for both drugs during as compared to after pregnancy (mean change 39% for cephradine and 31% for cefazolin). Half-lives of both drugs were significantly shorter during compared with after pregnancy (mean change 26% for cephradine and 35% for cefazolin). Consequently, total body clearance was increased during pregnancy. A significant negative correlation between length of gestation and total clearance per kg bodyweight was seen for cephradine. The bioavailability of oral cephradine did not differ significantly during compared with after pregnancy. It is concluded that the dosage of both cefazolin and cephradine should be increased when treating infections in pregnant women in order to obtain the same antibacterial effect as when treating non-pregnant women.