Anterior pituitaries were removed from female rats at various stages during the estrous cycle and FSH was fractionated by isoelectric focusing (IEF). Fourteen FSH components were observed during the estrous cycle and twelve of them were distributed between pH 3.71 and 6.66. IEF profiles of FSH in the pituitaries varied with the stage in the estrous cycle. Especially at the time of serum FSH surge on the day of proestrus, most of the components decreased, while only a highly alkaline component showed an increase. When these FSH components were separated and their nature was examined by radioreceptor assay (RRA), radioimmunoassay (RIA) and gel filtration, differences were observed among these components in the RRA/RIA ratio and gel filtration profile. As a general tendency, the RRA/RIA ratio of the components became greater while the apparent molecular size became smaller, as their pI became higher. However, some highly acidic components showed a biphasic elution pattern and the most acidic one eluted the latest on gel filtration, suggesting that these components may be heterogeneous in terms of molecular size. The FSH concentration in sera collected at different stages in the estrous cycle was measured by both RRA and RIA. The RRA/RIA ratio was high when the serum immunoreactive FSH was low, and low during the FSH surge. From these findings, it is concluded that the quality of FSH molecules present in the anterior pituitary gland changes dynamically throughout the estrous cycle, especially during the period of serum FSH surge. Furthermore it is suggested that the type of FSH secreted from it also varies according to the stage in the estrous cycle.