1. Growing pullets from two commercial strains were provided with saline (10 g sodium chloride) drinking water for three days to assess in vivo the urinary concentrating capacity of their kidneys. 2. Most of the pullets from strain A continued to gain body mass while drinking saline, indicating their kidneys conserved free water by forming a concentrated urine. Most pullets from strain B lost body mass while drinking saline, indicating their kidneys were unable to concentrate the urine sufficiently to obtain free water from the saline. 3. High proportions of large (0.23 to 0.42 mm circumference) glomeruli were found in the kidneys of pullets that gained body mass while drinking saline, whereas high proportions of small (0.07 to 0.18 mm circumference) glomeruli were found in the kidneys of pullets that lost body mass while drinking saline. 4. Glomerular sizes did not differ significantly when male birds of the two strains were compared. Urine from males of strain A had significantly (P less than 0.05) lower concentrations of hydrogen ions than urine from males of strain B, but no further differences were detected in comparisons of urine flow rates, glomerular filtration rates, renal plasma flow rates, urine osmolality, free water clearance, or sodium or potassium excretion rates.