As shown in a previous study [Schüpbach et al, 1984], different types of total and partial clefts of the secondary palate can be produced through amniocentesis performed in Sprague-Dawley rats at day 16.2 of gestation. Among these were a small number of unilateral clefts that were examined in the scanning electron microscope and in Epon sections. The 410 treated amnions yielded a total of 395 viable fetuses. Total clefts occurred in 48.9% of viable fetuses examined at day 17.8 and in 21.8% of those examined at days 19.3. Partial clefts were observed in 14.1% and 18.5% of viable fetuses examined at days 17.8 and 19.3, respectively. Unilateral clefts were observed in 3.8-10.2% of the partial clefts and in 0.5-1.8% of all viable fetuses. The eight animals with unilateral clefting included fetuses with a total unilateral cleft in the anterior hard palate. Morphological observations suggested that under conditions of delayed palatal closure total unilateral clefts may be the result of initial elevation of one, and delayed elevation of the other, shelf and partial unilateral clefts probably represent the result of an incomplete retrograde closure.