The effect of different families of fatty acids on the proliferation of rat uterine stromal cells (U I I I ) was studied. Docosahexaenoic acid (DHA) exerted a strong and dose-dependent inhibitory effect (IC 5 0 ~ 2 μM), whereas arachidonic acid (AA) stimulated U I I I cell proliferation at the optimal concentration of 10 μM. Oleic, linoleic and linolenic acids were ineffective from 0.1 to 10 μM. The inhibitory effect of DHA was independent of the eicosanoid biosynthesis and lipid peroxidation, since it was not reversed by the addition of the antioxidant BHT and no significant production of oxidized species from DHA occurred in our culture conditions.