The EMG characteristics of the normal auditory startle response in man are compatible with an origin in the pontine reticular formation and with conduction down the spinal cord in a slowly conducting, possibly reticulo-spinal pathway. The startle was reduced or absent in patients with progressive supranuclear palsy, consistent with loss of neurones in the lower pontine reticular formation. In contrast, the startle was present and of normal form in patients with Parkinson's disease. However, it was delayed in onset. This result was not influenced by treatment with L-dopa. The late auditory startle in Parkinson's disease might be related to withdrawal of facilitatory input to brainstem centres from the basal ganglia.