Suplphonylureas are known to stimulate insulin secretion from pancreatic β-cells by inhibiting ATP-sensitive K+ (KATP) channels. We have previously reported that the sulphonylurea drug tolbutamide also causes an increase in β-cell volume which is sufficient to activate volume-regulated anion channels (Best et al, 2004). In this study we examined the effects of sulphonylurea drugs on the volumes of both rat α-cells and β-cells. Islet cells were isolated as previously described from pancreatic tissue taken from rats which had been humanely killed by stunning and cervical dislocation (Miley et al, 1997). Cell volume was measured by a video-imaging method, and β-cells were differentiated from α-cells on the basis of cell size. Experiments were performed at 37°C in HCO3- -buffered solutions. With an extracellular glucose concentration of 10 mM, superfusion of β-cells with 100 μM tolbutamide (n=4); 0.1 μM glibenclamide (n = 4) or 10 μM meglitanide (n=3) caused significant increases in cell volume (P<0.05 by ANOVA). An increase in cell volume was also observed when α-cells were superfused with 100 µM tolbutamide (n=5; P<0.05) or 0.1 μM glibenclamide (n = 5; P<0.05) and 4 mM glucose. The mechanism by which cell volume is increased by the sulphonylureas was examined by studying α-cell volume regulation in response to hypertonic solutions in the presence of the sulphonylureas. In control experiments (absence of sulphonylurea, n= 7) and vehicle controls (0.1% v/v dimethylsulphoxide, n=4) the cells shrank in the hypertonic solution but did not exhibit a regulatory volume increase (RVI). By contrast in the presence of either 100 μM tolbutamide (n=6) or 0.1 μM glibenclamide (n=4) the cells shrank and then exhibited a RVI. The RVI observed in the presence of tolbutamide was inhibited by 10 μM eythylisopropyl amiloride (n=4). These data suggest that the sulphonylureas in α-cells may activate Na+-H+ exchangers which can contribute to increases in cell volume in both isotonic and hypertonic solutions. In conclusion, sulphonylureas have effects on ion transport in islet cells in addition to the inhibition of KATP channels.