Adipose conversion of cultured 3T3 cells is known to depend on an adipogenic factor present in serum. In the presence of this factor, extracts of different organs were found to inhibit the adipose conversion. The most active extracts were derived from brain, uterus, and pituitary, but other organs also possessed appreciable activity. Fibroblast growth factor partially purified from both brain and pituitary was much more active in suppressing adipose conversion than were crude extracts of the corresponding organs. Purified platelet-derived growth factor was also an effective inhibitor. Of all the tissue extracts tested, only pituitary possessed, in addition to the inhibitory activity, an adipogenic factor similar to that demonstrated previously in serum. This was revealed at concentrations of extract too low for the inhibitory factor to be effective. Under these conditions the pituitary extract had a specific adipogenic activity orders of magnitude higher than that of serum. We suggest that the adipogenic factor of serum may originate in the pituitary.