The objectives were to measure changes in fluid balance of mares at parturition and relate those changes to the foals' acquisition of passive immunity. Twelve Thoroughbred mares and their foals were observed for suckling behavior for the first 12h post-partum. The mare's water intake, packed cell volume, plasma protein concentration, and plasma osmolality were measured pre- and post-partum. The foals' immunoglobulin concentrations were measured at birth and 24h later. The median increase from pre- to post-foaling in mares' packed cell volume was 4.0% (interquartile range=0.3-6.8). The median increases in plasma protein and plasma osmolality after foaling were 0.9 (0.5-1.4)g/dL and 2.5 (0.8-6.0) milliosm/kg, respectively. Median daily water intake was 13.8 (7.6-20.8)L before and 42.1 (26.2-61.4)L 24h after foaling. The foals' IgG at 24h was significantly correlated with their dam's water intake in the 12h following parturition (r=0.66, P=0.039). The median duration of suckling during the first 4h of the foals' lives was 20 (5-85)min and 85 (42-220)min during the first 12h. The median number of suckling bouts was 34 (11-71) during the first 12h after birth.