AbstractThe objective of the present study was to determine if periods of exposure would modify the developmental toxicity of butyl benzyl phthalate (BBP). Pregnant Wistar rats were given BBP at a dose of 2.0% in the diet on days 0–20, days 0–7, days 7–16 or days 16–20 of pregnancy. Food consumption and body weight gain were decreased in the pregnant rats given BBP. All dams given BBP on days 0–20 exhibited complete resorption of all the implanted embryos. Post‐implantation loss in the pregnant rats given BBP on days 0–7 or 7–16 was higher than that in the control and pair‐fed pregnant rats. No increase in post‐implantation loss was found in the pregnant rats given BBP on days 16–20. Pre‐implantation loss in the BBP‐treated groups was comparable to the control and pair‐fed groups. Striking teratogenicity was detected in fetuses of the dams given BBP on days 7–16. Cleft palate and fusion of the sternebrae were predominantly observed. About 95% of the fetuses in this group had cleft palate. The incidence of malformations in this group was significantly and markedly higher than that in th