The nature of the stimulus(i) that initiates renal hypertrophy and hyperplasia is unknown. This investigation assays plasma of uninephrectomized rats for activity of renotropin(s), a humoral factor(s) which has been postulated to control compensatory renal growth. Plasma taken from rats 24 hours after uninephrectomy when compared to plasma taken from sham operated animals stimulates thymidine incorporation into DNA and uridine incorporation into RNA in kidney slices but not in liver slices. Stimulation is not seen in plasma from bilaterally nephrectomized rats. It is concluded that the assay described shows the presence of a circulating renal growth factor which is probably produced in response to a decrease in renal mass.