In 1-, 2- to 3-, 7- to 8-, and 10-day old intact and spinal rat puppies, studies have been made of the effect of l-DOPA (100 mg/kg, intraperitoneally) on autogenic periodic motor activity in the gastrocnemius muscle. In 1- to 3-day old pups, strong stimulating effect was observed up to a prolonged continuous activity for 5-10 or even more minutes. This effect decreases with age. Traces of the inhibitory effect are observed at early stages in the form of total decrease of the activity. The inhibitory effect increases with age. In all spinal animals, at the background of a decreased activity, stimulating effect of l-DOPA was predominantly observed. Age peculiarities of the effect of l-DOPA are discussed in relation to ontogenetic development of catecholaminergic innervation in rats. It is suggested that stimulating effect of l-DOPA is associated with its action on the descending noradrenergic system, whereas the inhibitory one is mediated by the brain structures.