In this study the influence of 6-hydroxydopamine (6-OHDA), in vitro and in vivo, on the oxygen consumption in the rat brain cortical slices was examined. The treatment with 6-OHDA increased the oxygen uptake of brain cortical tissue of young rats. The maximum increase was observed 7 and 14 days after treatment with 6-OHDA. On the contrary, 6-OHDA added in vitro produced very marked depression of oxygen uptake in slices of brain cortical tissue of the tested animals. The addition of isoprenaline in vitro stimulated the respiratory activity in the cerebral tissue of control young rats in all the periods of examination. Thus, 18 days after the birth, the isoprenaline-stimulation of oxygen uptake in brain tissue was 44.3% as compared to the control values. The same degree of stimulation was noted in the cerebral tissue of older animals (25, 32 and 45 days after birth). However, addition of isoprenaline did not influence the respiration of cerebral tissue stimulated by 6-OHDA.