This paper presents the design and fabrication of an implantable control unit intended for epidural spinal cord stimulation (ESCS) in rats. The device offers full programmability over stimulation parameters and delivers a constant current to an electrode array to be located within the spinal canal. It implements an adaptive voltage compliance in order to reduce the unnecessary power dissipation often experienced in current-controlled stimulation (CCS) devices. The compliance is provided by an adjustable boost converter that offers a voltage output in the range of 6.24 V to 28 V, allowing the device to deliver currents up to 1 mA through loads up to $25 \mathrm {k}\Omega $. The system has been fabricated using discrete components, paving the way to an inexpensive product that can easily be manufactured and batch produced. The control unit occupies a total volume of ~13.5 cm3 and therefore fulfills the size restrictions of a system to be implanted in a rat. Results indicate that by adjusting the voltage compliance a total power efficiency up to 35.5% can be achieved, saving around 60 mW when using lower stimulation currents or operating on smaller impedances. The achieved efficiency is the highest compared to similar stateof-the-art systems.