['Propionylcarnitine (PC) excretion has been measured during a clinical trial of metronidazole therapy in two patients with propionic acidaemia and two patients with methylmalonic aciduria. All patients were in good metabolic control and were receiving L-carnitine. While total propionate excretion was reduced by up to 40% in all four patients during metronidazole therapy, the excretion of propionylcarnitine remained largely unchanged. PC comprised up to 80% of total propionate excretion in patients with propionic acidaemia.'] Is propionylcarnitine excretion affected by metronidazole administration to patients with disorders of propionate metabolism?