Cadmium is a ubiquitous trace metal in the environment and a constituent of tobacco smoke. It is sequestered in the placenta, where it accumulates with advancing gestation, potentially inhibiting the synthesis and/or release of hormones produced by the trophoblast. It is also possible that cadmium competes with essential metals for binding to metallothionein, thus interfering with the transport of these micronutrients to the developing fetus. Experiments in our laboratory indicate that the production of p r o g e s t e r o n e by syncytiotrophoblast cells in vitro is inhibited by cadmium and that this inhibition may be due to a decline in low density lipoprotein-receptor transcription. Although syncytiotrophoblastic maturation does not appear to be affected by cadmium exposure, further studies will be needed to determine the metal's potential effect on cytotrophoblastic invasion of the uterine endometrium in early gestation.