OBJECTIVE To investigate the effects of lead exposure to rat placenta and pups during different gestation periods. METHODS All 108 Wistar rats (72 females, 36 males) were randomly divided into four groups. All rats were orally fed with 0.025% lead acetate during different gestation periods. Blood was obtained from the abdominal vena cava and the lead level in maternal blood was measured by means of atomic absorption spectrometry at the end of the pregnancy. The number of pups, their body weight, body length and tail length were measured. The effects of lead to rat placenta were observed by level of microscopy, optical microscopy and electronic microscopy. RESULTS Experimental groups the blood lead level at the end of gestation were above 0.483 micromol/L. There were significant differences among, of pups, during different groups (P < 0.01). Among them the drinking lead group of whole distant was the lowest in placenta weight [(0.31 +/- 0.13) g] body weight of pups [(2.08 +/- 0.88) g] length and tail length of pups [(2.37 +/- 0.32) cm, (0.98 +/- 0.09) cm]. There were significantly differences between the experimental groups and controls. Maternal blood lead level was negatively related to placenta weight (r = 0.652, P < 0.01), and had no relation with the body weight of pups (r = -0.107, P = 0.46). In the experimental groups of lead poisoned rats, the placenta showed focus necrosis in the deciduas, and increased the trophoblastic giant cells and light staining cells in the trophospongium. Trophoblast in the labyrinth and trophospongium showed degeneration; fibrin deposition around the villi was increased. Microvilli around the trophoblast were shorter and less, mitochondrion was swollen and decreased in number, rough endoplasmic reticulum was distended and ribosomal number on membrane decreased. CONCLUSIONS Lead exposure during different gestation periods should have a traumatic effect on the trophoblast, leading to interference of nutrition and oxygen exchange. Furthermore, the blood supply to the placenta and nutrition and oxygen exchange between mother and pups were also interfered, leading to reduction of placenta weight and retardation of development of pups.