Baseline plasma osmolality (pOsm) and plasma arginine vasopressin (pAVP) were measured in chronically catheterized fetal sheep. Mean baseline pAVP in fetuses from 101-120 days was 1.9 +/- 0.46 muU/ml (mean +/- SEM) with a pOsm of 293 +/- 1.8 mOsm/kg. In fetuses of 121-141 days of gestation, mean pAVP was significantly lower, 0.77 +/- 0.19 muU/ml (P less than 0.05), with a similar pOsm (293 +/- 1.9 mOsm/kg). The logarithmic baseline pAVP values were linearly correlated with pOsm for both groups. Hypertonic saline infusion resulted in a similar increase in the log pAVP corrected for the rise in pOsm in the 101-120-day fetuses and in 121-141-day fetuses. The slope of this response was similar to that of the steady state relationship. The data indicates that the fetal osmoreceptor system for control of arginine vasopressin secretion is fully functional in the last trimester of pregnancy.