Thermoregulation, metabolism and life history of species are affected by body size and shape. Based on specimens of the wood mouse Apodemus sylvaticus that were collected at Doñana National Park in 1978–81 and 2006–07, changes in body mass, body size, and allometry were tested between these periods. Furthermore, data from 1978–81, when more specimens were available, were used to evaluate the sexual dimorphism of adults. Between the two periods and regardless of age, the most striking reduction in size in both females and males concerned body mass (females ?29.5%, males ?36%) and ear length (?20% for both sexes). Although less pronounced (3–4%), also a significant reduction in the total cranial and the condyle-basal lengths of females but not of males was observed. No change was evident for the zygomatic width and the diastema length and for the head-body and hind foot lengths in either sex. The allometric relationships between the measured traits and the head-body length in adults did not change between the two periods. Males were larger than females in all the measured traits except the zygomatic width and the ear length. No sexual dimorphism was evident relative to the static allometry of adults. A major determinant of this reduction may have been a shortage in suitable resources. Overall, this study confirms and extends previous findings on male-biased sexual size dimorphism and reveals a dramatic decline in body mass, which is likely linked to the observed reduction in species abundance at Doñana. The extent and rapidity of the observed morphological changes raise concerns about the conservation of Doñana ecosystems and pose questions for future research on the ecological processes that caused these changes. informacion[at]ebd.csic.es: Docampo et al (2019) Marked reduction in body size of a wood mouse population in less than 30 years. Mammalian Biol https://doi.org/10.1016/j.mambio.2018.09.010