AbstractCells ofT. pyriformisstrain GL‐C synchronized in an amino‐acid‐free medium underwent synchronous oral replacement in place of cell division. This involved development of an oral primordium at the anterior end of the cell, with a concomitant rounding of the cell. The timing of this development was very similar to that of primordium development during synchronous division. In some experiments a second synchronous oral replacement was observed about 90 minutes after the first.Synchronization of oral replacement was similar to that of cell division. Prior to the heat shocks cells maintained in amino‐acid‐free medium were undergoing oral replacement asynchronously (with no concurrent macronuclear DNA synthesis). During the synchronizing treatment, development was initiated in the inter‐shock intervals but was arrested by the 34° heat shocks. As a result, cells became progressively accumulated in the earliest stage of oral replacement development. This accumulation did not take place if a single continuous heat treatment was given, or if cells were maintained in 2,4‐dinitrophenol, actinomycin D, or cycloheximide during the periodic heat shock treatment.The synchronous oral replacement which took place after the end of the synchronizing treatment (EST) could be blocked completely by actinomycin D, cycloheximide, and puromycin, and partially byp‐fluorophenylalanine. Exposure to cycloheximide (1 μg/ml) during stage 1 of stomatogenesis (10–30 minutes after EST) resulted in arrest of development, exposure during stages 3 and 4 (45 minutes) brought about primordium resorption, while exposure in stage 5 (60 minutes) permitted completion of development, with subsequent partial resorption. These responses were identical to those of synchronized dividing cells treated a