1. Lactose, sucrose or maltose were administered to rats by gavage together with 32PO4, and blood 32P was determined. 2. PO4 uptake into blood was increased only when lactose and PO4 were administered into the gut. 3. Weaned male rats were fed on a control diet or a diet containing 30, 60 or 120 g lactose/kg. After a 5 d period of adaptation, a 6 d P balance was carried out. After a further 24 d blood levels of PO4, and calcium and PO4 uptake by brush-border membrane vesicles prepared from the renal cortex, were determined. 4. The absorption and retention of P were higher in lactose-fed rats. Faecal P excretion decreased with increasing dietary lactose content. Urinary P excretion was lower in the rats fed on 30 and 60 g lactose/kg whereas the 120 g lactose/kg diet increased urinary P. 5. Membrane vesicles isolated from rats fed on the diets containing 30 and 60 g lactose/kg showed a higher uptake of PO4, but with the 120 g lactose/kg diet uptake was lower than with membrane vesicles isolated from rats fed on the control diet. 6. A statistically significant increase in the serum PO4 level was observed in the rats fed on the diet with 120 g lactose/kg. 7. The presence of lactose in the diet caused a stimulatory effect on PO4 absorption and retention.