Rat pups suspended in air and administered l-DOPA engage in a locomotor behavior termed air-stepping. The role of l-DOPA itself was investigated by administering several doses of an aromatic l-amino acid decarboxylase inhibitor, NSD 1015, prior to 100 mg/kg l-DOPA to 5-day-old rats. NSD 1015 dose-dependently increased the latency to onset and decreased the duration of l-DOPA-induced air-stepping. Thus l-DOPA induces air-stepping only after its conversion to dopamine and/or noradrenaline.