The present work examined some central nervous actions of prostaglandin D2 (PGD2), which is the most prevalent prostaglandin in rodent brain. The effects of PGD2 were compared with those of PGE2 and PGF2 alpha. The prostaglandins were administered intracerebroventricularly (ICV) to conscious rats using the method of Herman (1970). All three prostaglandins studied produced depressive behavioral effects, causing obvious sedation at doses of 2.0 micrograms and 20.0 micrograms ICV. PGD2 and PGE2 significantly reduced spontaneous motor activity at doses of 2.0 micrograms and 20.0 micrograms ICV. PGF2 alpha was less effective; only 20.0 micrograms significantly inhibited motor activity. At a dose of 20.0 micrograms ICV all three compounds were shown to block convulsions induced by pentylenetetrazol. PGD2, the most effective prostaglandin in this respect, was still slightly anticonvulsive at a dose of 2.0 micrograms ICV. PGF2 alpha hat the weakest anticonvulsive potency. PGE2 and PGF2 alpha (2.0 micrograms and 20.0 micrograms ICV) caused a marked hypertensive effect, whereas PGD2 at the same dose levels only produced a small increase in blood pressure. PGE2 and PGF2 alpha (2.0 micrograms and 20.0 micrograms) also exerted marked pyrogenic actions. The effects of PGD2 on body temperature were variable. When given at a dose of 20.0 micrograms ICV, it caused slight hyperthermia whereas a lower dose (2.0 micrograms ICV) induced a moderate fall in body temperature. These findings suggest a relationship between the actions of the different prostaglandins on blood pressure and body temperature.