The role(s) of one family of polypeptide growth factors in a developing organ system was examined. Renal anlagen (metanephroi) were surgically removed from 13-d-old rat embryos and grown in organ culture for up to 6 d. Over this period of time when placed in serum-free defined media, the metanephroi increased in size and morphologic complexity. Messenger RNAs for both insulin-like growth factors (IGFs), IGF I and IGF II, were present in the metanephroi. Immunoreactive IGF I and IGF II were produced by the renal anlagen and released into culture media. Levels were relatively constant during the 6 d in culture and averaged 3.5 X 10(-9) M IGF I and 8.3 X 10(-9) M IGF II in media removed from metanephroi after contact for 24 h. IGF binding protein activity was not detected in culture media. Growth and development of metanephroi in vitro was prevented by the addition of anti-IGF I or anti-IGF II antibodies to organ cultures. IGF II produced by metanephroi was active in an IGF II biological assay system and addition of anti-IGF II receptor antibodies to organ cultures prevented growth and development, consistent with the action of IGF II in metanephroi being mediated via the IGF II receptor. The data demonstrate production of both IGF I and IGF II by developing rat metanephroi in organ culture. Each of these peptides is necessary for growth and development of the renal anlage to take place in vitro. Our findings suggest that both IGF I and IGF II are produced within the developing metanephros in vivo and promote renal organogenesis.