A bolus for administering a biologically active material for example an insecticide into a liquid environment such as the water of a reservoir or lake or an anthelmintic into the rumen juices of an animal comprises a succession of liquid impermeable outer segments (153, 263) enclosing a core which in the simplest case may be a single unitary active material segment (151, Figure 14) or in a developed embodiment may comprise a succession of annular segments (269, Figure 25) contained in sealed recesses (267) in the outer segments (263) which are located on a degradable central rod (265) e.g. of magnesium alloy, which itself will be active for instance to combat magnesium deficiency. As shown in Figure 25, the device may incorporate a weight (275) which, in animal use, inhabits regurgitation. Additionally, the device may include a galvanic couple element which, with the rod (265) sets up a galvanic action which controls the speed of degradation of the rod (265) and thereby the rate of administration of the segments (26).