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The popularity of tattoos has increased tremendously in the last 10 yr particularly among athletes and military personnel. The tattooing process involves permanently depositing ink under the skin at a similar depth as eccrine sweat glands (3–5 mm).The purpose of this study was to compare the sweat rate and sweat Na+ concentration of tattooed versus nontattooed skin.The participants were 10 healthy men (age = 21 ± 1 yr), all with a unilateral tattoo covering a circular area at least 5.2 cm2. Sweat was stimulated by iontophoresis using agar gel disks impregnated with 0.5% pilocarpine nitrate. The nontattooed skin was located contralateral to the position of the tattooed skin. The disks used to collect sweat were composed of Tygon® tubing wound into a spiral so that the sweat was pulled into the tubing by capillary action. The sweat rate was determined by weighing the disk before and after sweat collection. The sweat Na+ concentration was determined by flame photometry.The mean sweat rate from tattooed skin was significantly less than nontattooed skin (0.18 ± 0.15 vs 0.35 ± 0.25 mg·cm−2·min−1; P = 0.001). All 10 participants generated less sweat from tattooed skin than nontattooed skin and the effect size was −0.79. The mean sweat Na+ concentration from tattooed skin was significantly higher than nontattooed skin (69.1 ± 28.9 vs 42.6 ± 15.2 mmol·L−1; P = 0.02). Nine of 10 participants had higher sweat Na+ concentration from tattooed skin than nontattooed skin, and the effect size was 1.01.Tattooed skin generated less sweat and a higher Na+ concentration than nontattooed skin when stimulated by pilocarpine iontophoresis.