Standard renal clearance techniques were used to compare the effects of D-glucose, L-glucose and D-mannitol on renal calcium handling and general renal function in the anaesthetized rat. A significant (P < 0.01) calciuresis was seen in animals infused with D-glucose. This resulted from a decreased tubular reabsorption of calcium. A similar response was not seen with L-glucose or D-mannitol. The contrasting actions of the three sugars on tubular calcium transport may relate to their different transport characteristics, since only D-glucose is actively reabsorbed. Responses of the three sugars also differed with respect to fractional fluid and sodium reabsorption which were significantly (P < 0.001) lower in animals infused with L-glucose and D-mannitol, and plasma insulin concentrations which were significantly (P < 0.01) elevated only in the D-glucose group. An unexpected finding was a highly significant (P < 0.001) and progressive fall in glomerular filtration rate (GFR) in animals infused with L-glucose. The reasons for this are not known.