Abstract: The formation of the synapton (synaptonemal complex) was followed by an electron microscopic examination of large samples of Saccharomyces cerevisiae cells at various stages of meiosis. Three temperature-sensitive mutants were used, cdc4, cdc5 and cdc7, which undergo a slow but normal meiosis at 25 degrees C. At the restrictive temperature of 34 degrees C, cdc4 and cdc5 arrest at an advanced enough stage of meiosis to allow the study of synapton morphogenesis. Based on the frequencies of nuclear structures, we describe the formation of the central region and central elements of the synapton in the dense body, which may be part of the nucleolus. This process occurs during early meiotic stages, concomittantly with recombination commitment and premeiotic DNA replication. Mature synaptons usually appear after premeiotic S, at the pachytene stage, and later disappear. A possible intermediate stage in this disappearance is found in arrested cdc5 cells, which contain paired lateral elements without central elements. Following the frequencies of spindle plaque configurations, we conclude that the plaques in meiosis duplicate once at the beginning of the main DNA replication, as is also observed prior to mitosis. In contrast to mitotic cells, however, meiotic plaques remain duplicated for a long period, until the synaptons disappear, and only then separate from each other to form a spindle. During late stages of the first meiotic division, the outer plates of the spindle plaques thicken, to duplicate later and give the second division spindles. The characteristically thick outer plate may have a role in the formations of the ascopore wall.