Monthly collections from October to April revealed seasonal alterations in the structure of the nucleoli of the microsporangiate strobili of the Scotch pine (Pinus sylvestris L.). By classifying the nucleoli in five types, two of them with subtypes, it was possible to list the amount and the sequence of the changes. The results indicated that nucleoli with fully intermingled fibrillar and granular components dominated in actively proliferating tissues whereas this type was almost lacking during the so-called dormant period. The wintertime nuclei, although remaining at interphases, also underwent structural alterations. In the early part of the winter, the most prominent feature was the loosening of the nucleolar structure, but this phenomenon decreased towards midwinter. The chromatin became strongly condensed and synchronous with this, the nucleolar organizer regions became visible as distinct, often ball-like entities. These were located at the surface of nucleoli, sometimes almost separated from them, and they were often partly, or fully, buried in condensed chromatin. Chromatin condensation relaxed transiently during the latter part of winter and at the same time, the granulation of the nucleoplasm increased. Nucleolar segregation disappeared before any mitoses began. The start of active cell divisions was accompanied by growth in the size of the nucleoli and by their increased stainability.