ExtractFetal intrauterine growth retardation was induced in nine rats at 17 days' gestation by ligating the blood supply to one of the uterine horns. The 27 fetuses from the uterine vessel ligated side were the intrauterine growth retarded fetuses (IUGR) and the 37 fetuses from the nonligated side were controls. The mean weight of the IUGR fetuses was 3.4 ± 0.3 g which was significantly smaller (P< 0.005) than the mean weight of the control fetuses, 4.0 ± 0.3 g.Whole body histidine decarboxylase (HDC) activity was assayed by a radioisotope method. The HDC activity per g tissue was not correlated to body weight in the control fetuses. This is in contrast to the IUGR fetuses in which the HDC per g tissue was significantly and inversely related to body weight (r− 0.48,P< 0.001).SpeculationIncreased histidine decarboxylase activity and histamine formation are generally associated with increased growth. Fetal rat intrauterine growth retardation was correlated with increased HDC activity. This suggests that the growth‐retarded fetuses were attempting to maintain growth by increasing HDC activity and histamine formation.