Magnesium (Mg) and zinc (Zn) levels were measured in amniotic fluid of 30 normal pregnancies between the 16th and the 42nd week of amenorrhea and in 15 cases of intra-uterine growth retardation (IUGR) at 30-38th week of pregnancy. While Zn levels were stable throughout physiological pregnancy, the Mg concentration decreased progressively from 1.09 +/- 0.31 mg/100 ml (mean +/- SD) at the end of the second trimester to 0.75 +/- 0.20 (p less than 0.02) at term. In the case of IUGR, Mg levels (1.22 +/- 0.28) were higher than in controls (0.89 +/- 0.27, p less than 0.01) matched for gestational age. No differences in Zn levels were found between the two groups. Although the mechanisms leading to Mg accumulation in the amniotic fluid of growth-retarded fetuses remain unknown, these data support the role played by Mg in fetal growth.