1. The authors investigated the effects of PGE1 on the action of sympathomimetics in the vas deferens of guinea-pig, comparing with those in the rat vas deferens, and also the action of PGE1 on the motility of nerve-free smooth muscle of chick amnion. 2. In the isolated guinea-pig vas deferens, the actions of phenylephrine and norepinephrine were much potentiated by pretreatment with PGE1. Futher, in the isolated hypogastric nerve-vas deferens preparation of guinea-pig, effects of phenylephrine, norepinephrine and tyramine on the contractile response of vas to the hypogastric nerve stimulation and to the transmural stimulation were also augumented especially in tension by PGE1-pretreatment. 3. In the isolated hypogastric nerve-vas preparation of rat, both contractile responses to hypogastric nerve and transmural stimulation were slowly reduced by treatment with PGE1 and the potentiated effect of phenylephrine or norepinephrine was not observed in spite of pretreatment with PGE1. 4. The actions of phenylephrine and norepinephrine on the denervated vas deferens of guinea-pig were also enhanced by PGE1 as it were in the intact vas deferens, but there was no significant effect by PGE1 on the action of norepinephrine in the denervated rat vas deferens. 5. PGE1 in low concentration (10-8g/ml) did not affect the spontaneous motility of nerve-free smooth muscle of chick amnion (9~11 th day incubated chick), but in large concentration (5 X 10-8g/ml) it caused irregular and slightly inhibitory movement. Pretreatment with PGE1 on chick amnion did not exert any change on the action of phenylephrine applied. However, the stimulatory action of physostigmine on the chick amnion was a little antagonized by the low concentration of PGE1. 6. It might be summarized that there is species difference between the actions of PGE1 on the vas deferens of guinea-pig and that of rat, and the action of PGE1 on the guinea-pig vas deferens might be mediated by the other mechanism rather than by direct action on the vas musculature.