Clearance experiments were performed with young pups born of guinea pigs given a daily injection of 4 mg of gentamicin per kg (body weight) from days 48 to 54 of gestation (term, 68 days). For 3-day-old animals, the glomerular filtration rate was similar to that measured in control guinea pigs of the same age whose mothers were given saline during the same period of gestation. The same applied to fractional excretion of water, urea, total solutes, Na, K, Ca, and Mg but not to fractional phosphate excretion, which increased significantly in the gentamicin group when compared with the controls (mean +/- standard error, 21.7 +/- 4.9 versus 7.3 +/- 1.8%; P less than 0.05; n = 6 for both). The glomerular volume of the juxtamedullary nephrons diminished by about 40%, and their proximal tubule length decreased by about 20%. The glomerular volume of the superficial nephrons also diminished, by about 30%, but their proximal tubule length did not change. The gentamicin concentration was higher in the renal cortex than in the medulla (13.1 +/- 2.6 versus 5.7 +/- 2.2 micrograms/g [dry wt]; P less than 0.01; n = 6 for each). It decreased significantly from days 3 to 20 in both tissues. No functional impairment of the kidney was found in 10-day-old animals, and normal or even supranormal morphometry of the nephrons was observed in the 20-day-old animals. It is concluded that fetal exposure to gentamicin impairs proximal tubular function in the developing animal and might also adversely affect glomerular and tubular growth. However, both the functional and morphometric impairments of nephrons are transitory.