The organization of the projection from the pretectal region to the inferior olive in the cat was studied with autoradiographic and horseradish peroxidase (HRP) methods. After injections of HRP into the olive in six cats, cells were labeled ipsilaterally in the anterior pretectal nucleus (NPA), the posterior pretectal nucleus (NPP), the nucleus of the optic tract (NOT), and the dorsal terminal nucleus of the accessory optic tract (DTN). In three experiments, tritiated amino acids were injected into those parts of the pretectal region which contained labeled cells in the HRP experiments, and the projections to the olive were plotted. Both NPA and NPP projected to the rostral half of the dorsal accessory olive, the rostromedial margin of the ventral lamella, and the lateral part of the ventrolateral outgrowth. NOT projected to the caudal half of the dorsal cap, while DTN projected to both the dorsal cap and nucleus beta. The projections are entirely ipsilateral.