The time-course of appearance of plasma vasotocin and the ontogeny of the vasotocin response to osmotic stimulation during embryonic development was examined in the chicken. In the first experiment, blood was obtained by heart puncture of chicken embryos on embryonic days (E) 15-20, and by decapitation of chickens on the day of hatching (D1 = E21). Plasma was separated for measurement of osmolality and vasotocin. Basal plasma osmolality remained constant until E20 (285-291 mosmol/kg) and was significantly increased at D1 (304 +/- 2 mosmol/kg). Plasma vasotocin was first detectable at E16 (5.5 +/- 0.8 (S.E.M.) pmol/l) and increased thereafter with a peak at E18 (21.8 +/- 2.3 pmol/l). The concentration decreased again at D1 (9.0 +/- 1.7 pmol/l). Osmotic stimulation with hypertonic NaCl 60 min before the animals were killed in the second experiment increased plasma osmolality, at all time points, by 9 +/- 0.2 mosmol/kg in all animals, but increased vasotocin in an age-dependent manner. The vasotocin response was greatest in D1 chickens, and the increase in vasotocin concentration relative to the control group value was 61 pmol/l, whereas the corresponding value in young (E15-E18) and older (E19-E20) embryos was only approximately 9 and 18 pmol/l respectively. In a third experiment the time-course of vasotocin secretion after osmotic stimuli in catheterized chickens (E18 and D1) was established. The basal plasma osmolality was significantly increased after 15 min in both groups. Vasotocin levels were significantly raised after 15 min in E18 and after 30 min in D1 chickens and further increased thereafter.(ABSTRACT TRUNCATED AT 250 WORDS)