The Slack gene encodes a voltage-dependent K(+) channel that has a unitary conductance of approximately 60 pS. Evidence from heterologous expression studies suggests that Slack channel subunits can also combine with the Slo subunit to generate Ca(2+)-activated K(+) channels of larger conductances. Nonetheless, the function of Slack in the brain remains to be identified. We have now generated an affinity-purified antibody against the N-terminal of rat Slack, for biochemical and immunohistochemical studies. The antibody recognized Slack in transiently transfected CHO cells both by immunocytochemistry and by Western blot analysis. The antibody also detected a single band in rat brain membranes. The localization of Slack in rat brain slices was then determined using the antibody. Most prominent Slack immunoreactivity occurs in the brainstem, in particular the trigeminal system and reticular formation, where very intense staining was found in both cell bodies and axonal fibers of associated nuclei. Labeling was also very strong in the vestibular and oculomotor nuclei. Within the auditory system, the medial nucleus of the trapezoid had a robust signal consistent with staining of the giant presynaptic terminals. Strong Slack immunoreactivity was present in the olfactory bulb, red nucleus, and deep cerebellar nuclei. There was labeling also in the thalamus, substantia nigra, and amygdala. The only cortical region in which Slack immunoreactivity was detected was the frontal cortex. The subcellular and regional distribution of Slack differs from that previously reported for the Slo channel subunit and suggests that Slack may also have an autonomous role in regulating the firing properties of neurons.