The influence of low humidity during incubation (LHI) on arginine vasotocin (AVT) secretion in male and female White Leghorn chicken embryos and chicks was investigated. LHI embryos were incubated from day 8 (E8) to E18 at a relative humidity of 32-35%, while control embryos were incubated at 58-62%. In the control groups, plasma AVT gradually increased from E15 to E18, increased sharply during hatching and returned to the E18 level within 4 h. LHI did not alter the level of AVT until E18. LHI E19 males had concentrations of AVT twice as high as those of control males (39 +/- 3.9 vs 15 +/- 1.6 pmol/l, P < 0.001). Augmentation persisted during hatching and 4 h afterwards. In LHI females additional elevation reached significant levels only at 4 h after hatching (58 +/- 13 vs 16 +/- 1.7 pmol/l, P < 0.01). Deprivation of water caused an increase in plasma sodium concentrations in the 2-day-old chicks of the control groups, while no significant changes were observed in the LHI group. Deprivation of water together with a high environmental temperature caused an additional increase in AVT only in the LHI groups (42 +/- 5.2 vs 22 +/- 3.9 pmol/l, P < 0.01 for males; 32 +/- 3.4 vs 19 +/- 2.8 pmol/l, P < 0.01 for females). The results clearly showed that LHI leads to a more pronounced response of the AVT system on subsequent osmotic and non-osmotic challenges, especially in males. Embryonal activation of the AVT system probably helps the organism to cope with hatching and posthatching stresses.