To assess the hydration status of 40 exclusively breast-fed Peruvian infants, their milk intake and urinary volume and concentration were measured during 8-hour daytime observations. Maximum home temperatures ranged between 26° and 33°C; environmental relative humidity ranged between 49% and 96%. The infants consumed between 105 and 528 gm milk during the observation period; the standardized intakes ranged between 4.0 and 12.1 gm/kg body weight per hour (mean±SD 7.5±1.7 gm). The infants volded between 0.9 and 6.3 ml urine per kilogram of body weight per hour (mean±SD 3.4±1.3 ml). The maximum urinary specific gravity in each infant ranged between 1.003 and 1.017. We conclude that healthy infants can maintain adequate hydration status while exclusively breast-fed under these environmental conditions. © 1986 The C. V. Mosby Company.