Dose regimens in perioperative fluid management are rarely evidence based. Therefore, we investigated responses to an IV fluid infusion in healthy volunteers to assess basic physiologic effects of a fluid infusion per se. In a prospective, double-blinded, cross-randomized study, 12 healthy volunteers with a median age of 63 yr (range, 59-67 yr) received an infusion of lactated Ringer's solution 40 mL/kg (median, 2820 mL) or 5 mL/kg (median, 353 mL; background infusion) in random order on two separate occasions. The study was designed to mimic the perioperative course with preoperative fasting, infusion of the fluid over 3 h in the morning, and additionally 24-h hospitalization under standardized conditions. Primary outcome assessments were pulmonary function (spirometry), exercise capacity (submaximal treadmill test), balance function (BalanceMaster), and weight. Infusion of 40 mL/kg of lactated Ringer's solution compared with the background infusion (5 mL/kg) resulted in a significant decrease in pulmonary function and a significant weight gain of median 0.85 kg (range, -0.2-1.6 kg; P = 0.003) persisting 24 h after the infusion. Exercise capacity and balance function were not influenced by fluid administration. These findings may serve as a basis for clinical studies applying the same type of fluid in different amounts to determine the optimal amount of perioperative fluid in various surgical procedures. IMPLICATIONS: Infusion of 40 mL/kg of lactated Ringer's solution in volunteers led to a significant decrease in pulmonary function and a significant weight gain for 24 h but without effects on exercise capacity. These findings may serve as basis information for clinical studies of perioperative fluid management.