1. Experiments were performed upon anaesthetized rats to investigate the effect upon the initial 4 h of ischaemic acute renal failure of intrarenal flush with phosphate buffer (PB), sucrose alone and phosphate-buffered sucrose (PBS) solutions of differing compositions.
2. Kidneys flushed with PB immediately before 45 min pedicle clamp ischaemia exhibited a post-ischaemic function similar to that of control, non-flushed kidneys: they were isosthenuric and non-oliguric with a clearance of inulin (Cin) depressed to 10% of the pre-ischaemic value.
3. Kidneys flushed with 50 mmol/l sucrose in saline before ischaemia became isosthenuric, oliguric and had a more severely depressed post-ischaemic Cin compared with non-flushed kidneys.
4. Kidneys flushed with PBS containing either 50 (PBS50) or 140 (PBS140) mmol/l sucrose at pH 7 became polyuric after ischaemia, produced concentrated urine and had Cin values significantly higher than control kidneys.
5. Raising the pH of the PBS50 to 7.4 resulted in isosthenuric, polyuric kidneys that had a post-ischaemic Cin not significantly different from that of the control group.
6. It is concluded that the severity of the initial phase of ischaemic acute renal failure was significantly reduced by intrarenal flush with a PBS solution of pH 7; flush with PB or sucrose alone had no protective action.