Pivampicillin (630 mg/kg body wt) given daily by stomach tube induced carnitine deficiency in the rat. The carnitine concentrations after 24 days were significantly reduced to (mean +/- SD) 34 +/- 2, 27 +/- 7, 70 +/- 18, 75 +/- 16 and 49 +/- 4% of controls in plasma, liver, muscle, heart and kidney, respectively, without any further reduction after 36 days. Pivampicillin treatment reduced the carnitine concentrations in the liver of the 48 hr fasted rat to about 1/2 of the controls after 6 days. The concentration of beta-hydroxybutyrate was significantly reduced up to 14 days of treatment, and again increased. There was no significant difference in the free fatty acid concentrations between treated and control rats. Thus, pivampicillin treatment induced carnitine deficiency in the rat, but not as pronounced as seen in humans. This is possibly caused by adjustment of bacterial flora in the gut or altered renal mechanisms. The pivampicillin-treated rat, therefore, is not a useful model for pronounced carnitine deficiency in humans.