The ultrastructure and morphometric features of nucleolar fibrillar centers have been studied in three different plant species over a long period of gametogenesis, extending from premeiosis to after the first postmeiotic mitosis, with respect to variations in nucleolar activity which occur throughout this process. Fibrillar centers represent the structural counterpart of the transcriptionally inactive portions of the nucleolar organizing region (NOR); in some gametogenic stages, no discrete fibrillar centers can be observed and, therefore, the structure of the whole NOR has been studied. Two clearly differentiated types of fibrillar center structure are described, with a close relationship to different stages of nucleolar activity. Stages with high activity show “homogeneous” fibrillar centers, whose chromatin is decondensed, and stages with low activity show “heterogeneous” fibrillar centers with both condensed and decondensed chromatin. These two types had previously been characterized cytochemically by us in somatic cells. Medium-active stages show fibrillar centers “intermediate” between the two main types. The morphometric behavior of fibrillar centers is also related to activity. The number of fibrillar centers per nucleolar section increases with the nucleolar activity, while their size decreases. However, the location of fibrillar centers in the nucleolus does not follow any special pattern, except in middle meiotic prophase I, in which the whole NOR occupies a peripheral position. This is due to a special NOR activity at these stages, whose products are stored as “nucleolar dense bodies.” All these data allow us to conclude that fibrillar centers are highly dynamic structures, extremely sensitive to changes in nucleolar activity. This work has been supported by the CAICYT Grant 3753/79 C2 and by the CSIC Grant 41123