A fully integrated 5 mHz highpass filter for electrophysiological signal acquisitions is described. Unlike the traditional Gm-C or pseudo-resistor architectures, the proposed continuous-time filter employs the current steering technique, which helps decrease the capacitance area and reduce the total harmonic distortion (THD) simultaneously. This design was fabricated in 0.18 μm CMOS technology. Experimental results indicate that it achieves -3 dB frequency band at 5 mHz, power consumption of 15 μW and input referred noise of 3.8 μV rms . The -80-dB THD was measured at 1-Hz 0.2-V p-p sine wave input.