Human ether-á-go-go-related gene (hERG) potassium channels have voltage-dependent closing (deactivation) kinetics that are unusually slow. A Per-Arnt-Sim (PAS) domain in the cytoplasmic N-terminal region of hERG regulates slow deactivation by making a direct interaction with another part of the hERG channel. The mechanism for slow deactivation is unclear, however, because the other regions of the channel that participate in regulation of deactivation are not known. To identify other functional determinants of slow deactivation, we generated hERG channels with deletions of the cytoplasmic C-terminal regions. We report that hERG channels with deletions of the cyclic nucleotide-binding domain (CNBD) had accelerated deactivation kinetics that were similar to those seen in hERG channels lacking the PAS domain. Channels with dual deletions of the PAS domain and the CNBD did not show further acceleration in deactivation, indicating that the PAS domain and the CNBD regulate deactivation by a convergent mechanism. A recombinant PAS domain that we previously showed could directly regulate PAS domain-deleted channels did not regulate channels with dual deletions of the PAS domain and CNBD, suggesting that the PAS domain did not interact with CNBD-deleted channels. Biochemical protein interaction assays showed that glutathione S-transferase (GST)-PAS (but not GST) bound to a CNBD-containing fusion protein. Coexpression of PAS domain-deleted subunits (with intact C-terminal regions) and CNBD-deleted subunits (with intact N-terminal regions) resulted in channels with partially restored slow deactivation kinetics, suggesting regulatory intersubunit interactions between PAS domains and CNBDs. Together, these data suggest that the mechanism for regulation of slow deactivation in hERG channels is an interaction between the N-terminal PAS domain and the C-terminal CNBD.