In order to estimate the influence of occlusion and sweating on forearm and vulvar skin surface water loss (SSWL), both were measured simultaneously and continuously for 30 min in 8 healthy women. Vulvar SSWL decreased significantly during the measuring period from 24.9 +/- 5.2 gm-2 h-1 (mean +/- standard error of the mean) in the first 5 min, to 13.4 +/- 1.7 gm-2 h-1 in the last 5 min (p less than 0.05), whereas no significant changes were observed in forearm SSWL. The vulvar SSWL decay curve followed a logarithmic equation of the form y = a*tb. Irregular SSWL increases ('bursts') were observed in vulvar (but not in forearm) skin of 7 out of 8 women. These SSWL bursts were considered to be caused by sweating. The study shows possible causes of systematic errors in vulvar irritation studies. Methods for error reduction are discussed.