Human intracranial microwire recordings have typically had poor signal-to-noise ratios (SNRs), often below 10 dB. The physiological signal source is a fixed-amplitude one; thus, SNR must be improved by reducing either noise or interference. An understanding of the interference sources, how they are coupled to the recording system, and their relative magnitudes is needed to improve SNR. We measured potentially interfering sources in a controlled laboratory model of microwire recordings. Specifically considered were interference from power lines, fluorescent lights, radio transmitters, and other nearby electrical devices. In the presence of typical mismatches in impedance (100 kohm) and loop area (30 cm2), the greatest sources of interference are capacitive coupling to power lines (11.4 microV(rms)), capacitive coupling to fluorescent lights (9.7 microV(rms)), and nonpower line capacitive interference (8.6 microV(rms)). The model and techniques employed here to study human microwire recordings may also be applied to other neurophysiological recordings.