Slow-release devices retained in the rumen, are a simple method for continuous administration of bioactives to ruminant animals. To satisfy regulatory requirements and avoid waste of bioactive due to under- or over-dosing, it is advantageous to have a constant and predictable release rate. Existing intraruminal controlled-release technologies cannot easily be adapted for different bioactives or rates of release and can be influenced by the variable physiological environment in the rumen. Some existing commercial products use the pressure generated by a hydrogen gas-producing cell to extrude fluids from a syringe-like device. This technology may provide advantages for ruminal controlled-release as the gas production rate is unaffected by environment in the rumen and can be easily adjusted using electrical resistance applied to the gas cell. This technology was adapted for use in the rumen in these studies. Initial experiments identified the need for greater understanding of the rate that hydrogen is produced by the gas cell and the rate that gas diffuses through the barrel walls. Gas production rate was found to be inversely proportional to the resistance applied to the gas-producing cell. Factors affecting gas diffusion rate from the device were studied and a polymer was identified that reduced hydrogen diffusion to 5% of that for the initial components used. A relationship was developed to predict the release profile of a device. Controlled-release devices were constructed from selected materials. They released blank formulation at in vitro at a constant rate, which was within experimental variation of predicted values. Release rates from the devices used in vivo were slightly higher than predicted. The presence of rumen gases inside in vivo devices suggested that the difference may be due to inward diffusion of these gases; these may be eliminated by further study of barrel materials. Recommendations on the redesign of this technology for use as a generic intraruminal delivery system are given.