Summry—The formation of secretion granules has been studied in the Golgi apparatus of granular epithelial cells of frog urinary bladders maintained at room temperature or cooled at 4°C for various lengths of time. In control animals, the Golgi apparatus was composed of the following stacked elements: subjacent to thecis‐rmelement mad up of anastomosed tubules, two elements in themid‐compartment consisted of flattened saccules interconnected by tubules. On thetrans‐face, two or three sacculo‐tubular elements were slightly dilated by an electron dense granular material. In thetrans‐Golgi elements, this material was segregated into dilatations of various sizes and shapes which are continuous with flattened portions devoid of stained material. In thetrans‐Golgi region, free irregular progranules, seemingly formed by rupture of thetrans‐most Golgi elements. In granular cells examined after 4 h at 4°C, all Golgi compartments were affected by the low temperature. Thecis‐half portion of the Golgi apparatus consisted mainly of anastomosed membranous tubules and thecis‐element was no longer recognizable. Thetrans‐compartment was reduced to a few flattened saccules with progranules hardly visible on theirtrans‐aspect. At later time intervals, there was a progressive reconstitution of thecis‐zone while saccular elements started to pile up in thetrans‐compartment. At 24 h, thetrans‐compartment comprised six to eight saccular elements which showed irregular dilatations filled with granular material separated by large flattened portions. These various observations were interpreted as indicating that thetrans‐compartment was a dynamic struct