This study presents a tool and images to help identify the estrous cycle stages in mice. The estrous cycle in mice consists of four stages: proestrus, estrus, metestrus, and diestrus, which repeat every 4-5 days. Identifying the stage of the estrous cycle is important for timed mating and pseudopregnancy. Visual observation of the vaginal opening is the quickest method, but can be difficult due to strain differences in coat color. Vaginal cytology is considered the most accurate method for identifying all stages, but is labor-intensive. An identification tool is provided to aid in determining the stage of the estrous cycle based on the type and proportion of cells in the smear. The tool is a visual representation of cell types and their relative proportions during each stage of the estrous cycle. The tool helps users determine the stage of the estrous cycle by placing an imaginary arrow on the chart and moving it clockwise until the cell types and proportions match. The study also presents images of the vaginal opening at each stage of the estrous cycle for three mouse strains: black (C57BL/6J), agouti (CByB6F1/J), and albino (BALB/cByJ). These images and descriptions are an excellent resource for learning how to determine the stage of the estrous cycle by visual observation or vaginal cytology. The study also describes the visual method for identifying estrous stages, which is quicker and less labor-intensive than vaginal cytology. The visual method involves examining the vaginal opening for specific characteristics. The study also describes the vaginal cytology method, which involves collecting vaginal cells and examining them under a microscope. The study found that the visual method is effective for identifying proestrus and estrus, but more challenging for metestrus and diestrus. The vaginal cytology method is recommended for accurately identifying all stages of the estrous cycle. The study also describes how to determine if ovulation occurred by checking for a vaginal plug. The study concludes that the estrous cycle identification tool is a useful resource for identifying the stage of the estrous cycle in mice.This study presents a tool and images to help identify the estrous cycle stages in mice. The estrous cycle in mice consists of four stages: proestrus, estrus, metestrus, and diestrus, which repeat every 4-5 days. Identifying the stage of the estrous cycle is important for timed mating and pseudopregnancy. Visual observation of the vaginal opening is the quickest method, but can be difficult due to strain differences in coat color. Vaginal cytology is considered the most accurate method for identifying all stages, but is labor-intensive. An identification tool is provided to aid in determining the stage of the estrous cycle based on the type and proportion of cells in the smear. The tool is a visual representation of cell types and their relative proportions during each stage of the estrous cycle. The tool helps users determine the stage of the estrous cycle by placing an imaginary arrow on the chart and moving it clockwise until the cell types and proportions match. The study also presents images of the vaginal opening at each stage of the estrous cycle for three mouse strains: black (C57BL/6J), agouti (CByB6F1/J), and albino (BALB/cByJ). These images and descriptions are an excellent resource for learning how to determine the stage of the estrous cycle by visual observation or vaginal cytology. The study also describes the visual method for identifying estrous stages, which is quicker and less labor-intensive than vaginal cytology. The visual method involves examining the vaginal opening for specific characteristics. The study also describes the vaginal cytology method, which involves collecting vaginal cells and examining them under a microscope. The study found that the visual method is effective for identifying proestrus and estrus, but more challenging for metestrus and diestrus. The vaginal cytology method is recommended for accurately identifying all stages of the estrous cycle. The study also describes how to determine if ovulation occurred by checking for a vaginal plug. The study concludes that the estrous cycle identification tool is a useful resource for identifying the stage of the estrous cycle in mice.