The observation that ipsiversive circling follows unilateral lesions of the deep layers of the superior colliculus (DLSC), combined with the recent demonstration of an ipsilateral inhibitory GABAergic projection from substantia nigra pars reticulata (SNr) to the DLSC suggests a role for tectal GABA in circling behavior. In the present experiment, GABA, the GABA agonist muscimol, and the GABA antagonists picrotoxin and bicuculline were injected into the DLSC through chronic cannulae. GABA and muscimol produced significantly higher ipsiversive circling and bodily asymmetry than saline injections. Picrotoxin and bicuculline resulted in significantly higher contraversive circling and asymmetry than saline injections. All drugs except bicuculline produced dose-dependent circling. GABA injections were also made into the mesencephalic reticular formation (MRF) and the periaqueductal gray (PAG). The MRF injections produced the same degree of circling and asymmetry as the DLSC injections. The PAG injections resulted in significantly lower amounts of circling than the DLSC GABA injections, but they resulted in equivalent measures of asymmetry. These results demonstrate that DLSC GABA produces circling and asymmetry, and suggest that the DLSC as well as the MRF serve as output stations for the expression of circling behavior initiated at the striatum.