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 an elastic distendable membrane, chamber having a fluid outlet. The apparatus also includes a highly novel fluid flow indicator that provides a readily discernible visible indication of fluid flow through the apparatus. Additionally, the apparatus includes a novel adjustable flow rate control means for precisely adjustably controlling the rate of fluid flow from the reservoir of the device.