Inward rectifier potassium (Kir) channels serve important functional and modulatory roles in a wide variety of cells. While the activity of several members of this channel family are tightly regulated by intracellular messengers such as adenosine triphosphate, G proteins, protein kinases and pH, other members are tonically active and activity is controlled only by the expression level of the protein. In a number of Kir channels, sequence motifs have been identified which determine how effectively the channel is trafficked to and from the plasma membrane. In this report, we identify a number of trafficking determinants in the Kir4.2 channel. Using mutational analysis, we found that truncation of the C terminus of the protein increased current density in Xenopus oocytes, although multiple mutations of the C terminus had no effect on current density. Instead, mutation of a unique region of the channel significantly increased current density. Selective mutation of a putative tyrosine phosphorylation site within this region mimicked the increase in current, suggesting that tyrosine phosphorylation of the protein increases channel retrieval from the membrane (or prevents trafficking to the membrane). Mutation of a previously identified trafficking determinant, K110N, also caused an increase in current density, and combining these mutations caused a multiplicative increase in current, suggesting that these two mutations increase current by independent mechanisms. These data demonstrate novel determinants of Kir4.2 channel expression.