The temporal organization of physiological and behavioral states is controlled by circadian clocks in apparently all eukaryotic organisms. In the cockroach Leucophaea maderae lesion and transplantation studies located the circadian pacemaker in the accessory medulla (AMe). The AMe is densely innervated by GABA-immunoreactive and peptidergic neurons, among them the pigment-dispersing factor-immunoreactive circadian pacemaker candidates. The large majority of cells of the cockroach AMe spike regularly and synchronously in the gamma frequency range of 25-70 Hz due to synaptic and non-synaptic coupling. While GABAergic coupling forms assemblies of phase-locked cells, in the absence of synaptic release the cells remain synchronized but fire now at a stable phase difference. To determine whether these coupling mechanisms of AMe neurons which are independent of synaptic release are based upon electrical synapses between the circadian pacemaker cells the gap junction blockers halothane, octanol and carbenoxolone were employed in presence and absence of synaptic transmission. Here, we show that different populations of AMe neurons appear to be coupled via gap junctions to maintain synchrony at a stable phase difference. This synchronization via gap junctions is a prerequisite to phase-locked assembly formation via synaptic interactions and to synchronous gamma-type action potential oscillations within the circadian clock.