- Behavioural effects of myo-inositol have been demonstrated in animal models of psychiatric disorders and in clinical trials in psychiatric patients. These effects of myo-inositol were thought to be consequences of its being a substrate for the enzyme phosphatidylinositol (PI) synthase. Recently, epi-inositol, an unnatural stereoisomer of myo-inositol that is not a substrate for PI synthase, was found to also have effects similar to those of myoinositol to reverse lithium-pilocarpine seizures. The present study measured the behaviour of rats in an elevated plus-maze model of anxiety after chronic treatment of 11 daily i.p. injections of epi-inositol, myo-inositol, or control solution. Epi-inositol reduced levels of anxiety-like behaviour in rats compared with controls, and its effect was stronger than that of myo-inositol. The increased relative effect of epi-inositol could be due to slower metabolism or to a different mechanism of action.