Pancreatic beta-cells express imidazoline binding sites which play a role in the regulation of insulin secretion, but it is not known whether ligands for these sites also affect other aspects of beta-cell physiology. In the present study, we have investigated the effects of a range of imidazoline reagents on the growth and viability of clonal pancreatic beta-cells (RINm5F and HIT-T15). Three imidazoline compounds (idazoxan, phentolamine and antazoline) were found to cause marked inhibition of beta-cell growth in a time and concentration dependent manner. Idazoxan was the most potent of these with as little as 0.5 microM causing a significant decrease in beta-cell viability (EC50 approximately 10 microM). All three imidazolines also decreased the viability of clonal beta-cells in parallel with their inhibitory effects on cell growth. These effects were not reproduced by any of a wide-range of other imidazoline compounds, including effective insulin secretagogues such as efaroxan and RX821002. The effects of the three ligands did not correlate with their relative potencies for binding to any of the well-characterised imidazoline binding sites nor to alpha2-adrenoceptors. In addition, the inhibitory responses were not antagonised by other imidazoline binding site ligands. The inhibitory effects of idazoxan on the growth of RINm5F and HIT-T15 beta-cells required as little as 3-h exposure to the imidazoline and were not readily reversible when the reagent was removed. Reductions in growth rate were accompanied by marked alterations in the morphology of the cells, which could be detected before loss of viability. Cells exposed to phentolamine showed the characteristic features of apoptosis in that the nuclei were condensed (as judged by acridine orange staining) and electrophoresis of DNA revealed the presence of oligonucleosomal fragmentation. These changes could not be detected in cells exposed to idazoxan despite the more profound reduction in viability induced by this agent. We conclude that a sub-group of imidazoline compounds can exert profoundly detrimental effects on the growth and viability of clonal beta-cells but that these effects do not correlate with their binding affinity at imidazoline binding sites or alpha2-adrenoceptors.