Current applied near the midline brainstem elicits rapid ipsiversive circling. Pontine and midbrain sites were stimulated concurrently with paired pulses and the number of pulse pairs required to produce 3 complete circles in 10 s was measured at various intrapair (C-T) intervals. The same results were obtained when C pulses were presented via the midbrain electrode and T pulses via the pontine electrode, or vice versa: as C-T interval increased from 0.4 to 2.0 ms, the number of pulse pairs required decreased gradually. These decreases occurred at longer C-T intervals than the refractory period decreases observed in single-electrode tests. These results imply that collision occurred in the directly stimulated axons and that a longitudinal bundle of uninterrupted axons mediates the circling behavior. The sites from which collision was obtained overlap with both medial longitudinal fasciculus and crossed tectobulbar and tectospinal tracts. The axons mediating circling appear to have conduction velocities from 2 to roughly 20 m/s. By comparison, superior colliculus units antidromically driven from contralateral electrode sites that produce circling had conduction velocities from 0.7 to 40 m/s.