Abstract: The invention concerns a device for dispensing an active substance to be used in a biological medium, in particular in the rumen of an animal, comprising: a sealed receptacle (1, 21, 41, 71, 81, 100, 120, 130) for containing the said substance, and having at least one opening; means for provisionally closing the opening of said receptacle, said closing means being maintained closed by at least one biodegradable element (13, 23, 45, 55, 65, 78, 89, 109, 113, 125, 135), such that subsequent to the biological degradation of said element, the closing means completely releases said opening.