Gating firmware and software were developed for the microPET II small animal scanner. The measured cardiac and respiratory signals were collected and converted to TTL gating signals by a Biopac MP150 data acquisition system and sent to microPET II through two BNC connectors on the front panel. During acquisition, the coincidence monitor takes the average of the last eight gate input cycles and inserts this into the list mode data stream on the falling edge of the gating pulse. This value is then used to determine the current time interval of the next gate cycle when the list mode data are sorted into sinograms. The gating firmware and software were validated by an experiment using a rotating point source. Mouse heart (¹⁸F-FDG) and bone (¹⁸F⁻) imaging was performed with simultaneous cardiac and respiratory gating. It was clearly demonstrated that the contractile function of the mouse heart can be studied by cardiac-gated imaging with microPET II. The left ventricular volumes at different times of the cardiac cycle were measured and the ejection fraction was calculated. In the bone scan, no detectable movement caused by heart contraction was observed. Respiratory motion was more subtle with virtually no motion for more than 75% of the respiratory cycle. The motion of the mouse heart and bones in the thorax caused by respiration was less than 1 mm. It appears with the current resolution of PET, and the small fraction of the respiratory cycle in which motion occurs, that respiratory gating is probably not necessary for most mouse cardiac studies.