Delayed reaction time and diminished vigilance have been observed in Parkinson's disease (PD) but have not been related to biochemical changes in the disease. As norepinephrine is reduced in the brain in PD and this neurotransmitter has been related to attention and learning, we investigated the relationship of neuropsychological measures, including those of reaction time and vigilance, to the norepinephrine metabolite 3-methoxy - 4 - hydroxyphenethyleneglycol (MHPG) in 39 patients with idiopathic PD. The MHPG levels correlated with a measure of general intellectual ability and with performance on reaction time and continuous performance tasks. Our data suggest that altered norepinephrine metabolism may contribute to some aspects of intellectual dysfunction in PD.