Nonpregnant and pregnant guinea pigs in the last third of gestation were injected intramuscularly with 4 mg of gentamicin per kg, and drug concentrations in plasma were determined by radioimmunoassay at several intervals after injection. The maximum gentamicin concentration was lower in pregnant than in nonpregnant animals (14.6 +/- 0.7 micrograms/ml versus 21.6 +/- 0.7 micrograms/ml), and the peak time occurred significantly later (0.57 +/- 0.12 h versus 0.13 +/- 0.02 h). Four hours after gentamicin injection, drug concentrations in plasma were 2.1 +/- 0.8 and 0.3 +/- 0.1 micrograms/ml in pregnant and nonpregnant animals, respectively. Pregnant animals therefore eliminated the drug from their plasma more slowly. These data provide good evidence that the kinetics of plasma gentamicin varies in pregnant females because its volume of distribution was larger in pregnant than in nonpregnant animals. Detectable but small amounts of gentamicin (less than or equal to 0.50 microgram/ml) were found in the plasma of 46 of 57 fetuses. However, no net variations in these concentrations were observed during the period between 15 min and 6 h after injection to the mother. Gentamicin concentrations were also determined in the kidneys, liver, lungs, heart, and brain of fetal guinea pigs after administration to their mothers of one daily injection of 4 mg/kg for 7 days. Gentamicin was present in all these fetal organs; however, as in the adult organs, the kidneys contained far more than any of the others. Gentamicin concentrations were not significantly different in the kidney cortex and medulla (1.79 +/- 0.16 versus 1.48 +/- 0.92 micrograms/g), indicating that, contrary to what is observed for adults, renal accumulation of gentamicin in the fetus does not occur preferentially in the cortex.