An apparatus for accurately infusing medicinal agents into an ambulatory patient at specific rates over extended periods of time. The apparatus is of a compact, low profile, laminate construction and includes either an elastic distendable membrane, or a thin barrier member which, in cooperation with a thin planar base defines a fluid chamber having a fluid outlet. The apparatus includes an internal stored energy source which functions to controllably expel the medicinal agents from the apparatus. The stored energy source may comprise either a distendable elastomeric membrane of an expandable sponge like cellular mass.