Rats were observed in enriched colony environments following radio-frequency lesions of the locus coeruleus (LC), zona compacta of substantia nigra (SN, or control operations. LC-lesioned animals were initially inactive, stayed in the burrows, and fell when climbing ropes and ramps. SN-lesioned rats were opposite to LC animals in many ways. They were hyperactive, had minimal motor disturbances, and were hyperaggressive. SN animals self-isolated and were not social-groomers, whereas LC rats socially-groomed and mounted other animals more than controls. These results provide evidence that the dopaminergic nigrostriatal pathway and the noradrenergic fibers innervating the cerebral cortex and limbic forebrain exert opposed effects upon behavior.