Zinc concentration in amniotic fluid and its relation to fetal weight were investigated in three groups of pregnant rats: group A received a zinc-adequate diet, and the rats of groups B and C were fed a zinc-deficient diet. Group C also received zinc supplementation in water. The daily food consumption, weight and plasma zinc levels on days 1, 10 and 20, zinc concentration in amniotic fluid, the number of implantation sites, the number of resorptions, the number of live fetuses and fetal weight were determined. Plasma zinc concentrations were significantly different at the end of gestation between group B (Zn = 167.6 +/- 26.6 micrograms/dl) and the other two groups (group A = 199 +/- 18.6 micrograms/dl; group C = 204 +/- 13.7 micrograms/dl). The number of resorptions was significantly higher in group B and the number of live fetuses was significantly lower in this same group (p less than 0.025). Fetal weight was significantly lower in group B (p less than 0.001). The zinc concentrations of the amniotic fluid were significantly lower in group B (14 +/- 4.7 micrograms/dl) as compared to group A (83 +/- 11.4 micrograms/dl) and C (82 +/- 21 micrograms/dl; p less than 0.001). There was a positive linear correlation between zinc concentrations in amniotic fluid and fetal weight, being r: 0.7379 (p less than 0.001).