The new N-methylnucleoside named doridosine, isolated from the shell-less marine dorid nudibranch Anisodoris nobilis, has been identified as 1-methylisoguanosine (1) by its spectral properties and by synthesis via methylation of isoguanosine. Doridosine has been shown to be identical to the nucleoside isolated from the Australian sponge Tedania digitata by Quinn, Gregson, Cook and Bartlett. They have also proved their product to be 1-methylisoguanosine. Doridosine was shown to cause prolonged reduced arterial pressure and reduced heart rate in the rat. The action is qualitatively similar to that of adenosine but of much greater duration, possibly because doridosine is resistant to attack by adenosine deaminase.