Treatment of 17 children aged 2-9.5 years with a combination of pivmecillinam and pivampicillin (250-500 mumol 24 h-1) for more than 1 year resulted in a reduction of the free carnitine concentration in serum and muscle to less than 10% of the mean reference value. The decline in serum was slow, with an estimated half-life of about 5 months. Spontaneous replenishment occurred at about the same slow rate. Thus, there is no increase in endogenous carnitine synthesis as a response to increased demand of carnitine for detoxification. Supplementation with carnitine during treatment required at least a four-fold molar excess over pivalic acid to achieve and sustain a normal carnitine concentration. The replenishment of carnitine occurred with a half-life of 1.1-3.0 months. From determination of muscle-carnitine concentration in patients treated with pivaloyl-containing antibiotics and in patients with organic aciduris, we conclude that serum carnitine is a good predictor of carnitine stores in the body. Six non-supplemented patients with a serum free-carnitine concentration of 0.7-2.6 mumol l-1 had an inadequate ketone-body increase during a 24-h fast. Vomiting, nausea and tiredness occurred in three cases following the fasting period. After normalization of the serum-carnitine concentration, a normal response to fasting was observed. Thus, in some organic acidurias, for example medium-chain acyl-CoA dehydrogenase deficiency, a low liver concentration of carnitine may be an important contributing factor to hypoglycaemic and Reye-like attacks. We believe that prodrugs which contain pivalic acid should be avoided if acceptable alternatives exist. If used, supplementation with at least four-fold molar excess of carnitine is advisable.