An implantable infusion device (10) includes a lightweight, inert housing (12) which contains an infusate (18) for administration to a patient, and an enclosed, gas-impermeable variable volume chamber (30) which contains a pressure source, such as propellant, mechanical spring, or the like. The variable volume chamber may be in the form of a rigid-walled expandable bellows structure or a nonstretchable flexible bag and is attached to the housing so as not to obstruct the entry port for introduction of infusate. A recharging fluid flow path (31) is provided adjacent to where the variable volume chamber is attached to the housing. The infusate is delivered from the device via a catheter in response to expansion of the variable volume chamber against the volume of infusate in the housing. A separate bolus injection port (38) is provided which allows infusate to be safely introduced directly into the catheter and overrides the controlled pressure-driven delivery. The lightweight housing may include integrally formed needle stops (32 and 34) and suture fastening loops (36).