Context: Sunscreen lotions are important to protect the skin during outdoor exercise, but they may interfere with sweating. Objective: To measure the effect of 2 water-resistant sunscreens on local sweat production in men and women exercising in the heat and to compare those effects with the expected inhibition resulting from the use of an antiperspirant. Design: Randomized crossover study. Setting: Exercise in the heat (ambient temperature = 30.2 degrees C 6 0.4 degrees C dry bulb and 58% +/- 4.3% relative humidity) in a controlled-environment laboratory. Patients or Other Participants: Twenty physically active, apparently healthy college students, 10 men (age = 22.5 +/- 2.8 years, height = 1.771 +/- 0.069 m, mass = 70.2 +/- 11.0 kg) and 10 women (age = 22.2 +/- 3.2 years, height = 1.625 +/- 0.075 m, mass = 57.7 +/- 7.9 kg). Intervention(s): With sweat-collection patches applied to their right and left scapular regions, the participants performed 2 exercise sessions on consecutive days. We assigned skin treatments (antiperspirant; organic chemical sun filter, sunscreen A; inorganic physical sun block, sunscreen B; no lotion) randomly to side and session. Participants pedaled at 79% 6 1% of maximum heart rate for 20 minutes in the heat. Main Outcome Measure(s): Scapular localized sweat rate. Results: No baseline, environmental, or exercise condition was different among skin treatments. Scapular localized sweat rate was lower for the antiperspirant treatment (88.3 mu L/min.dm(2); 95% confidence interval [CI] = 82.0, 94.7 mu L/min.dm(2)) and the inorganic physical sun block (sunscreen B) treatment (99.3 mu L/min.dm(2); 95% CI = 93.1, 105.5 mu L/min.dm(2)) than for the organic chemical sun filter (sunscreen A) treatment (114.8 mu L/min.dm(2); 95% CI = 108.8, 120.6 mu L/min.dm(2)) or the no-lotion treatment (122.6 mu L/min.dm(2); 95% CI = 116.2, 129.0 mu L/min.dm(2); P<.01). Conclusions: The inorganic physical sun block, sunscreen B, hindered effective sweating to the same extent as the antiperspirant, whereas the treatment with the organic chemical sun filter, sunscreen A, was not different from the control treatment. At this stage, it is not possible to identify the specific ingredient responsible for the effect.