An ultra-low-power capacitance-to-RF (C/RF) converter for laboratory mice blood pressure monitoring is proposed. Unlike the conventional design involving capacitance-to-analog (C/A) conversion followed by analog-to-digital (A/D) conversion, the proposed front-end is a direct capacitance-to-digital (C/D) converter that can simplify the hardware while saving both power and area. The C/D converter also features an automatic capacitance-range finder mapping the input capacitance range to the full scale of the digitization core. The generated digital data is compressed before driving the back-end RF transmitter, which is based on a power-ON/OFF VCO with direct FSK modulation operated at the 915-MHz ISM band. Optimized in 65-nm CMOS, the simulated 8-bit 6.4-kSa/s C/RF converter exhibits a 7.5 effective number of bit (ENOB) and a ~0.7 Vpp output swing at the RF transmitter output, while drawing 2.93 uW of power. The DNL and INL are +/-0.125 and +/-0.188 LSB, respectively. The attained capacitance sensing resolution is equivalent to 1.25 fF/LSB.