There has been recent interest in measuring sympathetic sudomotor function by autonomic surface potential analysis. The purpose of the present study was to assess factors affecting the reproducibility of the test. We determined the within-day and between-day reproducibility in 24 healthy volunteers. We used an increasing rather than a constant electrical stimulus to minimize habituation. The amplitudes were still highly variable (an average within-day coefficient of variation in the soles of 35%). Habituation did not, however, affect the latencies of the responses, which were much more reproducible (an average within-day coefficient of variation in the soles of 8%). Studies of between-day reproducibility revealed that the mean amplitudes were lower on day 2 vs. day 1 (0.706 +/- 0.10 vs. 0.85 +/- 0.10 mV in the soles, P less than 0.01) but the mean latencies were similar on the different testing days (2.09 +/- .04 seconds for the soles on day 1 vs. 2.16 +/- .05 seconds on day 2). We also assessed the sensitivity of surface potential analysis and report the results of testing 35 patients with far advanced autonomic neuropathy.