Optical-resolution photoacoustic microscopy (OR-PAM) has become a popular tool in small-animal studies. However, previous OR-PAM techniques variously lacked a high imaging speed, a high spatial resolution, and/or a large field of view. Here we report a high-speed OR-PAM system using an innovative water-immersible polygon-mirror scanner, which has achieved a cross-sectional frame rate of as high as 1200 Hz over a 12-mm scanning range. Using this polygon-scanner-based OR-PAM system, we have performed various studies on mouse models with stroke and cardiac arrests. We expect that the new OR-PAM system will become a powerful tool for imaging hemodynamics and neuronal functions.