The purpose of this study was to determine whether a simple noninvasive sweat collection method using skin patches would be useful in monitoring sweat Ca and to determine changes in dermal Ca loss during a bed rest study testing a resistive exercise countermeasure. The study showed that the technique was highly reproducible: the mean intra-subject variation approached zero and the inter-individual variability (%CV) varied from 18% to 32% for the three anatomical regions (arm, chest, and back) tested. There was less than 10% difference in sweat Ca excretion from different skin regions within the same individual at a given time point. A calculated estimate of total body sweat excretion for 12 bed rest subjects was 35 +/- 4 mg/day (mean +/- SE), close to published whole body measurements. Bed rest testing showed no significant differences with or without exercise when conducted in a temperature-controlled environment. We conclude that the skin patch technique is useful for monitoring changes in sweat Ca.