We report expression system-dependent effects of heterozygous mutations (P769L and A1059S) in the Cav3.2 CACNA1H gene identified in a pediatric patient with chronic pain and absence seizures. The mutations were introduced individually into recombinant channels and then analyzed by means of electrophysiology. When both mutants were coexpressed in tsA-201 cells, we observed a loss of channel function, with significantly smaller current densities across a wide range of voltages (−40 to +20 mV). In addition, when both mutant channels were co-expressed, the channels opened at a more depolarizing potential with a~5-mV right shift in the half-activation potential, with no changes in half-inactivation potential and the rate of recovery from inactivation. Interestingly, when both mutants were co-expressed in the neuronalderived CAD cells in a different extracellular milieu, the effect was remarkably different. Although not statistically significant (p < 0.07), current densities appeared augmented compared to wild-type channels and the difference in the halfactivation potential was lost. This could be attributed to the replacement of extracellular sodium and potassium with tetraethylammonium chloride. Our results show that experimental conditions can be a confounding factor in the biophysical effects of T-type calcium channel mutations found in certain neurological disorders.