Prostaglandins E1, B1, and F2α caused morphologic changes in L-929 cells and mouse embryo fibroblasts growing at low cell density similar to those produced by N6,O2'-dibutyryl cyclic adenosine monophosphate. The cell bodies elongated and the cellular processes were greatly extended. The prostaglandins also slowed cell growth and increased adhesion to the substratum. Since prostaglandins activate adenyl cyclase and raise intracellular levels of cyclic AMP, these effects were probably mediated through cyclic AMP. When confluent, the prostaglandin-treated L-929 cells resembled morphologically cells maintained in serumfree media. They lost their elongated cellular processes and became more round and flat.