Abstract: The present invention is concerned with improving the delivery of a drug from a ruminal delivery device to give a consistent delivery of drug to the ruminal environment. Thus, the invention is directed to an improved ruminal drug delivery device comprising a semipermeable membrane having an exit orifice and defining a compartment, the compartment containing a swellable osmotic agent expandable driving member, a drug to be dispensed, a density element and, optionally, a partition layer between the osmotic expandable driving member and the drug formulation, wherein the improvement comprises an essentially gas-impermeable barrier means that separates the density element from the other components within the delivery device for isolating gases evolved from the density element from the other components within the delivery device. The barrier means further includes a coupling to transfer osmotic pressure to the semipermeable wall.