Carbon-nanotube yarn (CNT-Y) made from high-purity, highly crystalized, double-walled carbon nanotubes is an advanced material with excellent electrical conductivity and flexibility; hence, it could potentially be used as a novel electrode for biopotential measurements. To our knowledge, the present study is the first in which CNT-Y electrodes were used to conduct electrocardiography (ECG) and electroencephalography (EEG) on experimental animals. All procedures and biopotential measurements were performed under isoflurane anesthesia. The CNT-Y electrodes were attached to the animals by creating a single interrupting suture on the skin. The lead II electrode configuration was used for ECG recording, i.e., the positive, negative, and body-earth electrodes were placed on the left apex of the auricular surface, the interscapular region, and the cervical region, respectively. The bipolar lead was used for EEG recording, with the exploring and reference electrodes on the bregma and base of the right auricular surface, respectively. Using CNT-Y electrodes, we obtained a clear ECG waveform from rats and a guinea pig; the QRS amplitude was ~1.4 mV. In rats, we obtained an EEG waveform with an amplitude of ~150 µV; the peak frequency was 0.8 Hz and the range was ~3 Hz according to power spectral density analysis. In the guinea pig, we obtained an EEG waveform with an amplitude of ~500 µV; the first peak was 0.1 Hz, the second peak was 1 Hz, and the range was ~3 Hz. These results show that CNT-Y could be used in toxicology studies to easily and inexpensively obtain high-resolution biological signals.