Few of the articles published on antibiotics and pregnancy are concerned with pharmacokinetics. It is particularly difficult to evaluate possible alterations in pharmacokinetic parameters that may be due to pregnancy. Most data available have been obtained in connection with abortion or delivery. Such data may not be representative for pregnancy as such. Marked changes in most organ systems, particularly in renal function, but in composition and amounts of body fluids as well, make it likely that several pharmacokinetic parameters change, possibly gradually as pregnancy progresses. Accumulated data for several beta-lactam antibiotics, and also for aminoglycosides indicate that antibiotics eliminated mainly by renal excretion will produce lower levels in serum or plasma in pregnant women than in other individuals. Also, the half-life of certain antibiotics in serum is shorter during pregnancy. Transplacental passage occurs for all antibiotics according to the physicochemical properties of the drug. Bolus injections to a pregnant woman are more efficient than continuous infusion in producing high levels of antibiotic in fetal serum and amniotic fluid. Fetal tissue levels are higher following multiple doses than after a single dose. Lower serum levels of antibiotics in pregnant women than in other individuals following the same dosage will be unsatisfactory as micr-organisms are less likely to be affected.