The effect of fasting on plasma concentrations of fatty acids has been determined in four children with medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. In addition, the in vivo rate of octanoate oxidation was measured, using [1-13C]octanoate. In the three older children (1.5-11.2 years), fasting for up to 18 h stimulated lipolysis, as reflected by the increasing concentration of free fatty acids, but with little rise in concentrations of medium-chain fatty acids, octanoate, decanoate and cis-4-decenoate. In an infant (0.5 year), lipolysis was greater and was accompanied by rising concentrations of medium-chain fatty acids. After 13.5 h there was a rapid increase in the concentration of decanoate and cis-4-decenoate. The calculated in vivo rate of octanoate oxidation was substantial in all patients studied (6.4-13.1 mumol/kg per h) despite very low MCAD activity in vitro. It is concluded that under basal conditions the in vivo oxidation rate of medium-chain fatty acids is near normal in the four children studied with MCAD deficiency.