1. Repaglinide, a novel compound with a nonsulphonylurea structure, is currently being clinically tested as a therapeutic agent. In the present study, the hypoglycaemic effects of repaglinide in rats and dogs were investigated. 2. Whereas the R-enantiomer, AG-EE 624 ZW, showed only weak hypoglycaemic activity, the S-enantiomer, repaglinide, turned out to be a potent hypoglycaemic compound in rats after oral as well as after intravenous administration. Only 50% of the dose of repaglinide was needed to be equieffective with the racemic mixture AG-EE 388 ZW. The corresponding ED50 values calculated for the effects after 120 min p.a. (intravenous administration) were 3.4 micrograms kg-1 (repaglinide) and 6 micrograms kg-1 (AG-EE 388 ZW). 3. When compared to glimepiride or glibenclamide, repaglinide displayed a 18 to 25 times higher potency in fasted rats. The ED50 values calculated for the effects after 120 min p.a. (oral administration) were 10 micrograms kg-1 (repaglinide), 182 micrograms kg-1 (glimepiride) and 255 micrograms kg-1 (glibenclamide). 4. In glucose loaded rats (0.5, 1.0, 2.0 and 3.0 g kg-1 glucose, p.o.) repaglinide exerted a very strong antihyperglycaemic activity which was even more pronounced than under normoglycaemic conditions. So for a reduction in blood glucose of 1 mmol l-1, 10.3, 9.3, 7.0 8.4 and 7.2 micrograms kg-1 repaglinide were needed after glucose loads of 0.0, 0.5, 1.0, 2.0 and 3.0 g kg-1, respectively. 5. In beagle dogs repaglinide again showed a pronounced hypoglycaemic effect (ED50 28.3 micrograms kg-1) which lasted for up to 24 h. However, insulin levels were only transiently increased. 6. The in vivo data presented are well supported by recently published in vitro findings. From its activity profile, repaglinide appears to be a promising new therapeutic agent.