AbstractSubcutaneous injection of pregnant mice on day 8 of gestation with 1,000 mg/kg or 200 mg/kg of an aqueous solution of the sodium salt of acetazolamide, divided into three equal treatments at 8‐hour intervals, in two series of experiments performed 8 months apart on separately purchased random bred CD‐1 animals, resulted in the occurrence of a novel variant of the frontal bones of the skull when animals exposed in utero were examined at 62 ± 2 days postnatal.In its greatest manifestation this variation consisted of a 2–3 mm posteriad extension of the dorsomedial portion of the posterior margin of both bones, although all degrees of asymmetry and variations in the extent of involvement of the posterior margin of the bones were seen. This frontal extension variant was seen in 30.9 ± 4.7% and 53.1 ± 5.0% of the 97 and 98 specimens in the highdose groups, and in 8.1 ± 2.7% and 14.6 ± 3.7% of the 99 and 89 low‐dose specimens in the two series. It was not seen in untreated groups, or to any significant extent in vehicle‐treated groups, or in a treatment regime involving exposure during days 9–11 of gestation. It was also not seen in 2,110 specimens from four other series of experiments reported in detail involving the teratogens, trypan blue (two series), 5‐bromodeoxyuridine, or 2,4,5‐T, or in approximately 1,000 specimens from comparable experiments with a variety of pesticides (Maneb, Captan, Trifluralin, Decamethrin) or Thalidomide.In contrast, a morphologically analogous variant, accessory parietal, in which a suture in the anterodorsal surface of the parietal bones running posteromedial and seemingly delimiting a separate bone, is seen in all series. Although it looks as though frontal extension could be an accessory parietal fused with the frontal bone, observation of the specimens does not strongly support that relationship. Even if that were the case, it happens only in day‐8 acetazolamide exposure.From the rather extensive series of observations performed, it would appear that frontal extension is a specific response to in utero exposure to acetazolamide during