A patient who had selective paralysis of downward gaze caused by bilateral lesions of the dorsolateral mesencephalic periaqueductal gray (PAG) matter is reported. Her necropsy findings differed from all previous reports of the syndrome, in that regions of the mesencephalon that have been considered as critical for executing downward gaze (dorsomedial to red nuclei, rostral interstitial nuclei of the medial longitudinal fasciculus [ri MLF]) were normal. These lesions may have produced the syndrome by involving the caudal portions of the nuclei of the posterior commissure (subcommissural), from which one of the commissural systems used by the ri MLF originates. It is also possible that the syndrome was produced by selective destruction of PAG neurons that generate downward impulses or by interruption of posterior commissure fibers containing downward impulses that travel through the dorsolateral PAG before terminating in the more ventral mesencephalon.