Skin conductance changes (SCC) reflect the activity in the sympathetic postganglionic cholinergic fibers, which innervate the sweat glands located in the palm of the hand and the sole of the foot. The purpose of this study was to measure the changes in the number of waves per second, the wave amplitude, and the mean skin conductance level during the 1st year of life. During SCC elicited by an auditory stimulus we measured the percentage of infants that responded, the amplitude, latency and recovery times, and any habituation pattern. Thirty-nine full-born, healthy infants were investigated during their 1st and 3rd days of life, at 3 and 10 wk of life, and at 6 and 12 mo of life. The mean skin conductance level (p < 0.001), the number of waves with an amplitude threshold higher than 0.5 and 1.0 micro siemens (p < 0.001), and the amplitude of the waves (p < 0.001) all increased during the first 10 wk of life. The percentage that responded to stimuli increased from 8% to 50%, and the amplitude of the response increased during the first 10 wk of life (p < 0.001). The level of arousal influenced the mean skin conductance level, the number of waves per second, and the amplitude of the waves during the 1st year of life. In conclusion, these results indicate that the part of the sympathetic nervous system associated with arousal develops during the first 10 wk of life.