1. Normal rat liver slices were incubated for 24 h in culture medium to which had been added freeze-dried plasma, prepared from the blood of rats which had undergone partial hepatectomy (60-70% of liver removed) 10 min, 4 or 24 h previously. No changes in the dry weight of the slices were observed. 2. Similarly, there were no changes in the dry weight or protein content of incubated liver slices when freeze-dried plasma prepared from the blood of rats unilaterally nephrectomized 10 min, 4 or 24 h previously was added to the culture medium. 3. Slices from the remaining liver stump, taken 4 h after partial hepatectomy (60-70% of liver removed) and incubated for 4 h in culture medium, did not release a factor capable of increasing the dry weight of normal liver slices incubated in the same medium. 4. After partial hepatectomy (60-70% of liver removed), there was no increase of guanosine 3':5'-cyclic monophosphate in the remaining part of the liver. 5. No measurable amount of esterase activity was found in normal liver or in the remaining liver stump after partial hepatectomy (60-70% of liver removed). 6. It is suggested that the stimulatory mechanism for liver regeneration is of a different nature from that involved in compensatory renal hypertrophy.