The 131I activity was measured in 30 human fetal thyroids in Zagreb district after the Chernobyl accident. A model of radioiodine metabolism in the mother and human fetus which takes into account the age dependence of the uptake and retention of radioiodine in the fetal thyroid was developed. Having assessed that the total intake by the average mother was about 1330 Bq, a good correlation between calculated and measured fetal thyroid activities was found (r = 0.77, P less than 0.001). The fetal thyroid dose reached the maximum of 0.43 micro Gy/Bq intake at about the fifth month of gestation. It was concluded that the risk of having a child with a harmful trait due to 131I absorbed by the mother was negligible.