An implantable hydraulic penile erectile device comprises a pair of occlusive cuffs (32 and 34) which are surgically implanted and wrapped around respective crura (18 and 20). The cuffs have expandable domes (56) on their inner surface (54). The domes are connected, via a check valve (44) and respective tubes (36, 38, 64, and 66), to a pump-reservoir (42). The pump-reservoir and the check valves are implanted in the scrotum. The system in general is filled with a working fluid so that when the pump-reservoir is squeezed, the working fluid is shifted to the expandable domes via the check valve. Expansion of the domes squeezes the crura of the penis and hence the deep penile veins (26a and 26b). This restricts venous blood flow from the cavernous bodies, causing engorgement of the spongy tissue and thus inducing erection. For deactivation of the device, the user squeezes the check valve so that bypass channels (68 and 70) are formed therein. Through these channels the fluid can return from the domes back to the pump-reservoir.