In all of our previous studies into the effect of amniotic sac puncture (ASP) carried out on day 13 of pregnancy in mice, we have used intraperitoneal Avertin (tribromoethanol) as the general anaesthetic. In the present study, we used an inhalational anaesthetic (a mixture of halothane, oxygen and nitrous oxide in a ratio of 2:3:3). The principal difference between these two regimens is that even under optimal post-operative conditions when Avertin is used it can take between 45 and 90 min before complete recovery is achieved; when the inhalational anaesthetic is used, complete recovery is usually achieved within about 3-5 min. Because the experimental conditions were otherwise identical, this allowed the influence of the anaesthetic employed during ASP and the incidence of abnormalities induced on survival rate to day 19 of pregnancy to be studied. The survival rate was slightly higher when the inhalational anaesthetic was used, as was the incidence of limb abnormalities, although the overall incidence of gross abnormalities involving the palate, limbs and tail was not significantly different. The most marked difference, however, was in the incidence of syndactyly, which was significantly lower when the inhalational compared to the intraperitoneal anaesthetic was used: 26.6% v. 70.2% of the abnormal limbs analysed. A possible hypothesis is presented to explain this difference.