Long-chain fatty acids (FA) have been shown to regulate expression of the gene for the adipocyte FA-binding protein aP2. We examined whether this effect was exerted by FA themselves or by a FA metabolite. The alpha-bromo derivative of palmitate, an inhibitor of FA oxidation, was synthesized in the radioactive form, and its metabolism was investigated and correlated with its ability to induce aP2 in Ob1771 preadipocytes. alpha-Bromopalmitate was not utilized by preadipocytes. It was not cleared from the medium over a 24-hr period and was not incorporated into cellular lipids. Short incubations indicated that alpha-bromopalmitate exchanged across the preadipocyte membrane but remained in the free form inside the cell. In line with this, preadipocyte homogenates did not activate alpha-bromopalmitate to the acyl form. However, although it was not metabolized, bromopalmitate was much more potent than native FA in inducing aP2 gene expression. Induction exhibited the characteristics previously described for native FA, indicating that a similar if not identical mechanism was involved. The data indicated that induction of aP2 was exerted by unprocessed FA. Finally, in contrast to preadipocytes, adipocytes metabolized bromopalmitate. This reflected increased activity with cell differentiation of a palmitoyl-CoA synthase that could activate palmitate and bromopalmitate at about one-fifth the rate for palmitate. In preadipocytes, the predominant fatty-acyl-CoA synthase, arachidonyl-CoA synthase, had very low affinity for both FA. Increased activity of the palmitoyl-CoA synthase, which has a wider substrate range, is likely to be important for initiation of lipid deposition.