The body condition of animals, as a proxy of individual fitness, is often assessed using the body condition index (BCI) and is important in evaluating life histories, reproduction, conservation resource management, and the impact of climate change. We will present the results of the first multi-species analysis of the BCI of small mammals in the middle latitudes, focusing on nearly 30,000 individuals from 18 species trapped in Lithuania between 1980 and 2023, comparing BCIs among species, and assessing variations based on age, gender, and reproductive status. We used a BCI calculated as BCI = (Q/L³) × 10⁵, where Q is the body weight in grams (the weight of the uterus with embryos excluded for pregnant females), and L is the body length in millimeters. Our results showed that seven of the eight species with the highest average BCI are granivores or omnivores, consuming animal food at least occasionally. Two distinct ontogeny patterns were observed: BCI either decreased or increased from juvenile to adult, likely due to dietary differences. Reproductive stress was found to negatively impact the BCI of adult females across all species and nearly all adult males. Additionally, two shrew species occasionally exhibited the Chitty effect, characterized by a high body mass and very high BCI, formerly not known for these species. This study enhances our understanding of how environmental changes affect small mammals and highlights the need for further analyses considering habitat dynamics, long-term variation in BCIs, and its value in relation to diet.