The mutant, sex-linked recessive px (paroxysm) gene, expressed in female White Leghorn chicks (Gallus domesticus), causes seizures beginning on approximately day 9 after hatching. In an attempt to determine possible central nervous system involvement in the seizures, brain levels of the putative transmitter noradrenaline were assayed. Brains of px chicks and controls (normal female siblings) were removed at 7, 14, and 21 days, weighed, and frozen immediately in liquid nitrogen for later analysis by the alumina-trihydroxyindoleamine method. Noradrenaline was assayed in whole brain and in 4 brain parts: optic lobes, brainstem, cerebral hemispheres, and cerebellum. No differences between px and controls were found in whole brain or optic lobe levels. Brainstem levels of noradrenaline increased significantly with time in both groups, though px increases were more rapid. Px chicks at 21 days of age had significantly higher levels or noradrenaline in the cerebral hemispheres, while cerebellar levels were significantly lower at the same age. The differences found in brainstem, cerebral, and cerebellar noradrenaline are of interest as possible explanations for the lowered body temperature, hyperexcitability, and lack of motor coordination seen in the px chicks.