The synchronization of MRI data acquisition to the respiratory cycle is crucial for collecting high-resolution images of small animals. Data collected during at-rest periods (breath-hold or between breaths) do not suffer from the blurring effects of respiratory motion seen in unsynchronized images. Here, we describe a simple, inexpensive, robust respiratory gating unit to achieve this synchronization. The unit can operate in either free breathing or driven breathing (ventilator) modes, with respiration detected optically (breathing motion) or via a pressure transducer. We demonstrate the effectiveness of this unit with in-vivo mouse images of lung and liver.