Thirty newly diagnosed patients with Parkinson's disease and 30 patients with primary depressive illness showed slowing of response on a computerized digit symbol substitution test when compared with 30 matched normal control subjects. Significant slowing was related, in the parkinsonian patients, to structural brain disorder and affective impairment and, in the depressed patients, to motor impairment. A second computerized test, cognitively simpler but requiring the same motor response, was also administered to each subject. Both cognitive and motor slowing seemed to contribute to slowing of response in the digit symbol test in both parkinsonian and depressed patients. The tests were repeated after about six months in 12 subjects from each group. The parkinsonian patients, on dopaminergic treatment, showed neither significant change in motor or affective impairment, nor improvement in response time for the digit symbol test, but change in response time was related to change in depression rating. The depessed patients, on conventional treatment, showed significant improvement in both affective and motor impairment and improvement in response time for the digit symbol test, due to improvement in cognitive slowing. It is proposed that bradyphrenia in Parkinson's disease and psychomotor retardation in depressive illness are closely related, and that impairment of dopaminergic systems may be involved in both.