As part of a study on red blood cell (RBC) senescence, we investigated whether the activity of endogenous Ca²⁺-sensitive K⁺ channels (Gardos channels) varied with RBC age. RBCs suspended in low-K⁺ media and selectively permeabilized to K⁺ via Gardos channel activation, rapidly dehydrate by the net loss of KCl and water. When RBC dehydration was followed in time by flow cytometry the Gaussian distribution of volumes was largely retained throughout dehydration (Lew et al. 2005), suggesting uniformity of dehydration rates in all cells. It is well documented that RBCs become denser with age. Therefore, during dehydration, old cells should stay ahead of young cells within the shifting Gaussian distribution. We tested this prediction using glycosylated haemoglobin, Hb A1c, as a reliable age-marker for normal human RBCs. RBCs suspended in a low-K⁺ buffer were Ca²⁺-loaded in conditions known to generate instant, uniform and maximal Gardos channel activation in the cells (Lew et al. 2005). Frequent samples were taken and spun through diethyl phthalate oil, with density 1.117 g/ml, a density not exceeded initially by any RBC. Replacement of 10 mM Cl^– by 10 mM SCN^– in the medium ensured full dehydration of all the cells within 3-4 min. In each sample we measured the percent cells with density above (dense) and below (light) 1.117 g/ml, and their Hb A1c fraction. Surprisingly, the cells harvested from the first pellets had the lowest Hb A1c (young cells) and those taking longest to become dense had the highest Hb A1c (old cells). This indicated that Gardos channel-mediated dehydration was much more vigorous in the younger RBCs and that retention of the overall Gaussian distribution concealed a substantial age-scrambling of the cells during dehydration. In conclusion, the results show that Gardos channel activity, measured at saturating Ca²⁺ loads, declines sharply throughout the life-span of human RBCs.