National AALAS 2024 -- Determination of the phases of estrous cycle in experimental rat studies is important for reproductive function studies and is required by different regulatory authorities. Vaginal smears are usually collected in the morning (e.g., between 08:00 to 12:00 at our facility). However, sample collections in the early morning could lead to missed proestrus stage and could be challenging when multiple studies are conducted at the same time in the same test facility. To minimize missed proestrus stage and to provide timing flexibility of sample collection, we evaluated the vaginal smear samples collected in the morning and afternoon in Sprague Dawley® and Wistar Han® rats.

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