The present invention is a composition for, and a method of, delivering an active agent at a controlled rate. The composition of the invention is a hollow ceramic or inorganic microtubule, where the active agent is contained within the lumen of the microtubules. Typically, the agent is adsorbed onto an inner surface of the microtubule. The method of the invention is disposing this novel composition in a use environment. In a preferred embodiment of the invention, a microtubule is a tubule having an inner diameter of less than 0.2 .mu.m, and microtubules are tubules having an average inner diameter less than 0.2 .mu.m. In a preferred embodiment, the hollow ceramic or inorganic microtubule is a mineral microtubule, such as halloysite, cylindrite, boulangerite, or imogolite. In a more preferred embodiment of the invention, the mineral microtubule has a biodegradable polymeric carrier disposed in its lumen. In a preferred embodiment of the invention, the inner diameter of the microtubules varies from about 0.20 .mu.m to about 0.35 .mu.m, or averages about 0.40 .mu.m. In another preferred embodiment, of the invention, the inner diameter of the microtubules varies from about 200 .ANG. to about 1000 .ANG..