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. Disposed within the fluid chamber is a thin fluid permeable member which precisely controls the rate of fluid flow through the 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 fill assembly comprising a prefilled vial that can be used to fill the fluid reservoir of the device with a selected medicinal fluid.