Based upon many investigations, the existence of short-lived, specific, circulating substances which incite and/or regulate compensatory renal growth has been proposed. In our studies, we find that sera and plasma from unilaterally nephrectomized rats compared to sera and plasma from sham-operated rats stimulate the incorporation of (3)H-thymidine monophosphate, (3)H-thymidine and (14)C-uridine into the DNA of incubating rat kidney fragments. While extracts from growing rat kidneys are not excitatory, they produce a relative enhancement to incorporation of isotope into DNA when combined with sera from uninephrectomized rats-more than the sera do alone. The above is found also for the incorporation of (14)C-uridine into RNA of incubating rat kidney fragments. Sera from uninephrectomized rats fail to stimulate DNA synthesis in liver slices from rats but do so in the presence of extracts from growing kidneys. Renotropic factors in sera and extracts do not appear to work by diluting the isotopes, by enhancing transport, or by effecting overall metabolism of the renal cells. The above described serum and liver factors may play a role in compensatory renal growth.