Objective: The aim of the present report was to determine the possible modifications in rat pituitary LH isoforms induced by the spontaneous increase in GnRH at the time of the preovulatory gonadotropin surge. Design: The changes in the quantitative pattern and relative proportions of pituitary LH isoforms in rats on the afternoon of proestrus [INT-P(PM)] were evaluated by comparison with other stages of the estrous cycle (diestrus-1, diestrus-2 and estrus) and ovariectomized (7 and 30 days earlier) animals killed in the morning and in the afternoon of the corresponding day. Methods: The chromatofocusing technique (pH gradient 11.00–7.00) was used to analyze the different molecular species of intrapituitary LH. Results: Pituitary LH from diestrus-1 animals, considered as a baseline pattern in the cycling rat, eluted as 11 isoforms distributed in pH 9.62–8.82, with greater percentages in pH 9.50–9.01. Except for INT-P(PM) pituitaries, there were no major differences in the pattern of LH heterogeneity in the pituitaries of rats from various stages of the cycle. In contrast, significant changes in the charge distribution and relative abundance of LH isoforms were found in the pituitaries from INT-P(PM) rats. INT-P(PM) pituitaries resolved in 16 LH isoforms with a significant shift to less alkaline pIs (pH 9.62–8.11), the more abundant being focused within pH 9.00–8.51. Conversely, a shift to more basic isoforms resulted after ovariectomy, leading to the accumulation of less mature isoforms in the gonadotrope. Conclusions: Presumably, the use of animals on INT-P(PM), at the time of the preovulatory LH surge, made it possible to discriminate such changes in LH isoform distribution. That GnRH, released in association with the rising phase of the LH surge, induces these changes in pituitary LH polymorphism appears to be the most likely possibility. In a previous study we demonstrated that GnRH stimulated galactose incorporation into LH in vitro. In the case of pituitaries from INT-P(PM) rats, the shift toward less alkaline isoforms could potentially result from sialylation of increased terminal galactose.