A fiber-optic Fabry-Perot interferometric medical pressure sensor of 125 μm in diameter and results of animal experiments are presented. A Fabry-Perot cavity is formed at an optical fiber end using MEMS (Micro Electro Mechanical Systems) technology. Deformation of the diaphragm of the sensor induced by pressure varies the Fabry-Perot cavity length. White light interferometry is used for detecting the change of cavity length to avoid noise caused by bending of the optical fiber and fluctuation of the light source. The spectrum of the modulated reflection light from the sensor interferometer is measured by high-speed spectrometer controlled by a micro-controller with parallel signal processing. The detection system tracks peak wavelengths of the modulated light and determines the sensor cavity length, which corresponds the applied pressure. Pressure changes have been monitored by using the developed detection system. The total rate of sampling at the spectrometer, data transfer from the spectrometer to the micro-controller, calculation and data output is 800 Hz. Animal experiments using a rat have been carried out and dynamic blood pressure changes in a carotid artery have been successfully monitored.