The developmental toxicity of acetonitrile and five halogenated derivatives was examined with an in vivo teratology screen adapted for use in the Long Evans rat. The screen was extended to an evaluation of growth till postnatal days 41-42, and weight of several organs at sacrifice. Acetonitrile was without developmental effects even at doses toxic to the dam. Of the halogenated compounds, treatment with trichloroacetonitrile (TCAN) and dichloroacetonitrile (DCAN) resulted in reduced fertility and increased early implantation failure. There was no effect on litter size in females bearing live litters, but pup birth weight was reduced in all litters exposed to halogenated compounds. Perinatal survival of the pups was adversely impacted by DCAN and TCAN. Postnatal growth till day 4 was reduced by DCAN and bromochloroacetonitrile (BCAN) while growth till day 42 was consistently affected only by TCAN. Some general observations were made on the usefulness of the criteria used in the screen, and TCAN, the most toxic of the halogenated compounds, was selected for further in depth evaluation.