The invention relates to a composition consisting of a dendrimer provided with blocking agents and an active substance occluded in the dendrimer. According to the invention a blocking agent is a compound which is sterically of sufficient size, which readily enters into a chemical bond with the terminal groups of a dendrimer and which can also be split off from the dendrimer or be modified without thereby affecting the chemical structure of the dendrimer and the active substance. The blocking agent can be provided with protective groups. Such conjugates have the advantage that the time at which the releasing of the active substance starts, can be controlled. The invention also relates to a process for the preparation of such a composition and to a process for the controlled release of the active substance.