Cardiac inward rectifier potassium current (IK1) plays a paramount role in repolarisation and stabilisation of the membrane potential in cardiomyocytes. Its downregulation contributes to arrhythmogenesis in heart failure and post-myocardial infarction. We have undertaken the molecular and functional characterisation of IK1 in the HL-1 cell line, which is derived from a mouse atrial tumour. HL-1 cells express many genes typical of differentiated adult cardiomyocytes and so far they are the only cardiac cell line able to divide continuously while maintaining a differentiated cardiac phenotype.¹ IK1 has not been described in HL-1 cells. Whole-cell recording was performed under conditions that have been shown to preserve IK1 in excised membrane patches; the pipette solution contained 10 mM pyrophosphate, 5 mM fluoride and 0.1 mM vanadate. Extracellular barium at an IK1-selective concentration (0.1 mM) inhibited inward current by 44%±4.2% (mean±SEM, n=13 cells), with little or no effect on outward current. The apparent reversal potential of the barium-sensitive current was −58±6.0 mV in 20 mM and −10±3.9 mV in 100 mM extracellular potassium with 160 mM potassium in the pipette (n=7). Reverse transcriptase PCR was performed with HL-1 complimentary DNA and primers specific for Kir2.1 (KCNJ2), the dominant IK1 isoform in cardiomyocytes. A PCR fragment of the expected size and sequence was produced. These results support the conclusion that HL-1 cells have an inwardly rectifying potassium current and express Kir2.1.