Hypotonic stress causes the activation of both K⁺ and Cl^– channels in cells, allowing ions to leave the cell followed by osmotically obliged water. The cell returns to its physiological volume, by a process known as regulatory volume decrease (RVD). There are two main types of K⁺ channels; voltage-gated and Ca²⁺-activated. Voltage-gated channels comprise the delayed rectifier and A-type channels while the Ca²⁺-activated channels comprise BK, IK and SK channels. Both channel types have been identified in neuronal cells, however the K⁺ channel responsible for RVD in CAD cells is not known. This study investigated the K⁺ channel activated in response to hypotonic stress using the whole cell patch clamping technique. K⁺ currents were recorded under isotonic and hypotonic conditions at 20^oC. Cells were held at a holding potential of -60mV for 4.64ms and were subjected to a step protocol (-60mV to +60mV in 20mV increments), each pulse lasting 100ms. In the presence of both intracellular and extracellular Ca²⁺ the K⁺ current increased from 20.13±1.9pA/pF to 33.06±2.3pA/pF (n=5) upon hypotonic stress. In the absence of extracellular Ca²⁺ the K⁺ current increased from 24.82±1.3pA/pF to 30.68±1.5pA/pF (n=5). Experiments in the absence of intracellular Ca²⁺ also showed channel activation with current increasing from 25.13±1.3pA/pF to 28.15±1.6pA/pF (n=4). Removal of both extracellular and intracellular Ca²⁺ resulted in a decrease in K⁺ current from 29.43±2.8pA/pF to 27.87±3.2pA/pF (n=3). Inhibition of the hypotonically induced current from 31.0±4.2pA/pF to 16.7±4.0pA/pF using the SK inhibitor apamin (100nM) suggested a role for this channel. The K⁺ current activated in the presence of both intracellular and extracellular Ca²⁺ was significantly greater than that observed in the absence of either extracellular or intracellular Ca²⁺ or the addition of apamin (ANOVA and Tukey HSD). Under Ca²⁺ free conditions a significant reduction in current is observed when compared to the current activated under control conditions, suggesting that whilst intracellular Ca²⁺ can activate the K⁺ channel, extracellular also plays a role. The removal of intracellular Ca²⁺ also reduced the amount of current through the channel while the removal of both intracellular and extracellular Ca²⁺ resulted in the abolition of channel activation. These results suggest that intracellular Ca²⁺ plays a greater role in channel activation compared to extracellular Ca²⁺. The abolition of current in the absence of Ca²⁺ and the reduction of current in the presence of apamin suggest the SK Ca²⁺-activated K⁺ channel is responsible for the K⁺ efflux following hypotonic exposure in neuronal CAD cells.