Osmotically driven drug delivery devices with walls of porous hydrophobic material as the sole diffusion-limiting wall component are disclosed. Despite pores which are considerably larger than the molecular dimensions of osmotically active species in the interior of the device, despite pores which by their hydrophobic character do not permit the passage of liquid water, and despite the absence of a cellulosic semi-permeable membrane, the devices absorb water by osmosis without loss of the osmotically active species, and function in a manner analogous to osmotic pumps of the prior art. By permitting the use of hydrophobic substances as the wall material, the invention permits the device to be constructed entirely from biodegradable materials.