In pregnancy, dehydration produces marked effects on maternal and fetal body water homeostasis including an increase in fetal urinary sodium concentration and excretion. To examine the role of fetal plasma atrial natriuretic factor (ANF) and glomerular ANF receptors in dehydration-induced natriuresis, we compared plasma ANF levels and glomerular ANF binding characteristics in dehydrated and control maternal and fetal sheep. Mean (± SEM) maternal and fetal plasma ANF levels in control animals (n = 9) at 132–136 days gestation were 37 + 3 pg/ml and 138 ± 20 pg/ml, respectively. Although mean ANF receptor maximum binding capacities (Bmax) were significantly higher in maternal than in fetal glomeruli (83 ± 11 vs. 34 ± 12 fmol/mg protein, respectively), the dissociation constants (Kd) for ANF binding were not different (2.7 ± 0.6 and 3.7 ± 1.7 × 10^––10M, respectively). In an additional 9 animals studied after 63 ± 4 h of water deprivation, maternal plasma ANF levels were significantly lower than in the control group (14 ± 4 vs. 37 ± 3 pg/ml), maternal glomerular ANF receptor Bmax values were significantly higher (732 ± 203 vs. 83 ± 11 fmol/mg protein), and Kd values were six-fold higher (17.0 ± 7.1 vs. 2.7 ± 0.6 × 10^––10M), although this difference was only marginally significant (p = 0.06). In contrast to the adult, there was a small, nonsignificant decrease in plasma ANF levels and no difference in Bmax or Kd values between the dehydrated and euhydrated fetal animals. These results indicate that the maternal ewe, like the adult rat, responds to dehydration with decreased plasma ANF levels and up-regulation of renal ANF receptors. The fetal lamb by contrast has increased plasma ANF levels and decreased renal glomerular ANF binding relative to the ewe, and fetal renal ANF binding characteristics are not affected by maternal dehydration. Thus, while changes in maternal plasma ANF and renal ANF receptors may partially explain maternal dehydration-induced natriuresis, the role of ANF in fetal dehydration-induced natriuresis remains unclear.