The electrical impedance of 1-year-old twigs of three hardwoods and conifers was measured at several frequencies before and after boiling. In all species the impedance before killing decreased as frequency increased from 100 to 40 000 c.p.s. The rate of decrease in conifers was greater than in hardwoods, particularly at low frequencies. After the twigs were killed, the drastically reduced impedance exhibited only a slight dependence on frequency. The variability relative to the mean was about the same at all frequencies.The contribution of xylem and bark (i.e., periderm, phloem, and cambium combined) to the impedance was determined by measuring the impedance of the two tissues together and separately. The importance of uniform electrode penetration was demonstrated.