This study investigated fluid and electrolyte balance in well-trained male and female swimmers during 2 training sessions. Participants were 17 nationally ranked swimmers measured during a period of intensive training. Sweat loss was assessed from changes in body mass after correction for fluid intake and urine collection. Sweat composition was measured from waterproof absorbent patches applied at 4 skin sites. Air and pool-water temperatures were 36 degrees C and 27.4 degrees C, respectively. Training lasted 105 min in each session. All measured variables were similar on the 2 testing days. Mean sweat-volume loss was 548 +/- 243 ml, and mean sweat rate was 0.31 +/- 0.1 L/hr. Mean fluid intake was 489 +/- 270 ml. Mean body-mass loss was 0.10 0.50 kg, equivalent to 0.1% +/- 0.7% dehydration. Mean pretraining urine osmolality was 662 +/- 222 mOsm/kg, which was negatively associated with both mean drink volume consumed (p = .044, r = .244) and mean urine volume produced during training (p = .002, r2 = .468). Mean sweat Na+, K+, and Cl- concentrations (mmol/L) were 43 +/- 14, 4 +/- 1, and 31 +/- 9, respectively; values were not different between males and females and were not different between days except for a marginal difference in K+ concentration. The average swimmer remained hydrated during the session, and calculated sweat rates were similar to those in previous aquatic studies.