Following boron intake, multiple effects have been observed in animal experiments. However, human data is lacking, and no data is available on the ability of boron to accumulate in fetal tissues. Positive responses in animal species suggest that developmental toxicity may be an area of concern in humans, following exposure to boron. Two hypotheses have seemed to account for the multiple effects described in scientific findings. One hypothesis is that boron is a negative regulator that influences a number of metabolic pathways by competitively inhibiting some key enzyme reactions. The other hypothesis is that boron has a role in ionic membrane transport regulations. To better understand boron potential toxicity, the present study examined the relationship between boron exposure and some key enzymes, well-known for their affinity for mineral elements, such as delta-aminolevulinic acid dehydratase (ALA-D), and two fundamental enzymes having a role in ionic membrane transport regulations (Ca-pump and Na(+)K(+)-ATPase). We investigated the potential effects of an environmental boron exposure on the activity of these enzymes in an urban population of 197 "normal" newborns. Environmental boron exposure was assessed in placental tissue. Because of the well-known inhibiting effect of lead on these enzymes, cord blood and placental lead were also analyzed. After adjustment for potential confounders, including lead, placental boron levels were negatively significantly correlated to ALA-D activity while Ca-pump and Na(+)K(+)-ATPase activities did not seem to be affected by the level of boron exposure. Given boron's ability, as a Lewis acid, to complex with hydroxyl groups, we suggest that such a mechanism would explain the inhibiting effect of boron on ALA-D.