Stress-susceptible (SS) pigs exhibit locomotor disturbances and catecholamine deficiencies characteristic of certain basal ganglia disorders. Because the neurochemical and neurocytological roles in the stress syndrome are still uncertain in the pig, this study was conducted to determine whether glutamic acid decarboxylase (GAD), synthesizing enzyme of neurotransmitter (gamma)-aminobutyric acid (GABA), was involved in the etiology of the porcine stress syndrome (PSS); to observe age effects on neuronal and synaptic morphometric characteristics of SS and stress-resistant (SR) pigs; and to analyze Golgi neuronal types of caudate nuclei (CN) in young pigs;GAD activity was measured in substantia nigra (SN), CN, putamen, hypothalamus, and cerebral cortex of both SS and SR pigs at two different ages by a microradioenzymatic method. Significantly higher GAD activity was found in SN of SS pigs than in that of SR pigs. Significantly lower GAD activity, however, was associated with the hypothalamus of the SS pigs. In other brain regions, there were no differences in GAD activity between SS and SR pigs. The findings of differences in GAD activity in the Sn and hypothalamus of SS pigs are suggestive of a GAD involvement in PSS. Developmental increases of GAD activity were also found in SN, CN, and putamen, but not in the hypothalamus and cerebral cortex;The CN from 14 SS and 14 SR pigs of three different ages were processed according to the routine histological procedures for neuronal morphometric study. Tissues were also prepared for synaptic density measurements and stained with ethanolic phosphotungstic acid;There were significant decreases of neuronal and synaptic density with age in the CN of SS and SR pigs. The magnitude of the neuronal density decrease was significantly greater in SS pigs than in SR pigs, while both SS and SR pigs had similar synaptic density decline with age. The SS pigs had a greater loss of small neurons than medium neurons during aging, whereas, the SR pigs had a greater loss of medium neurons than small neurons with age;A modification of the Golgi-Kopsch method was applied to study the porcine CN neuronal types. Four major spiny and three major aspiny neuronal types were distinguished in two week old pigs.