Institute of Physiology, Czechoslovak Academy of Sciences, Prague, Czechoslovakia, 142 20 (Received 18 April 1977) If Brattleboro rats, homozygous (DI) and heterozygous (non-DI) for diabetes insipidus are deprived of water, no difference in urinary osmotic pressure can be detected between the two genotypes before 14 days of age Dlouhá, Křeček & Zicha, 1976). This may arise from a shortage of vasopressin in non-DI rats until week 3 after birth, as has been demonstrated in rats of the Wistar strain (Křeček, Dlouhá & Křečková, 1961). We have attempted to detect differences in osmoregulation in DI and non-DI rats before diabetes insipidus is manifest. Samples of urine were obtained from 147 young Brattleboro rats by perineal stimulation. In 10- and 14-day-old animals, all procedures were done at 30 °C. Dehydration was induced in 10-, 14-, 18- and 22-day-old rats by water deprivation for 6 h; water diuresis was induced in 14-, 19-