Previous studies have demonstrated that the lateral spiriform nucleus (SpL) of the avian pretectum receives a major input from the ipsilateral basal ganglia (Karten and Dubbeldam, '73) and projects to the ipsilateral optic tectum (Brecha et al., '76). The present study has further detailed the anatomical organization of the afferent and efferent connections of SpL, with particular reference to (1) the sources of afferent inputs to SpL, (2) the projection targets of SpL, and (3) the laminar termination pattern of the SpL projection to the tectum. The SpL was found to receive clear-cut major inputs from only three nuclei: (1) the ipsilateral paleostriatum primitivum (PP) of the basal ganglia (the avian homologue of the mammalian globus pallidus), (2) the ipsilateral anterior nucleus of the ansa lenticularis (ALa) of the diencephalon, and (3) the ipsilateral nucleus tegmentipedunculopontinus (TPc) of the mesencephalon. Both TPc and ALa have previously been noted themselves to receive major inputs from the ipsilateral PP (Karten and Dubbeldam, '73). Two other cell groups may give rise to a slight projection to the ipsilateral SpL: (1) the posterior nucleus of the ansa lenticularis (ALp) of the diencephalon and (2) the nucleus semilunaris (SLu) of the isthmic brainstem. The ALp also receives a major input from PP (Karten and Dubbeldam, '73). Two other cell groups may give rise to a slight projection to the ipsilateral SpL: (1) the posterior nucleus of the ansa lenticularis (ALp) of the diencephalon and (2) the nucleus semilunaris (SLu) of the isthmic brainstem. The ALp also receives a major input from PP (Karten and Dubbeldam, '73), while SLu receives a major tectal projection (Hunt and Kunzle, '76a). The ipsilateral tectum was found to be the only projection target of SpL. The present data suggest that the SpL projection to the tectum is restricted to layers 8-13, with layers 11-13 receiving the heaviest projection from SpL. Among layers 8-10, layer 9 receives the lightest projection from SpL. The present results indicate that SpL receives only a limited number of inputs, which in all likelihood relay largely basal ganglia input to SpL. Since SpL projects only to the tectum, the sole function of SpL apparently is the transmission of ipsilateral basal ganglia influences to the avian optic tectum. Tectal layers 8-15 have been previously found to represent the layers of origin of the descending pathways of the avian tectum to hindbrain motor and "premotor" cell groups (Reiner and Karten, '82). In view of the purported involvement of the basal ganglia in motor functions, the basal ganglia pathway to the ipsilateral tectum via SpL may represent a major route by which the avian basal ganglia exert influences over motor functions.