Total brain size and the volume of several brain parts were compared in male and female ranch mink of varying age and body size in an attempt to quantify postnatal maturation and growth processes in this altricial species. Volumes of fresh whole brains and of different brain parts were calculated from prepared histological sections from juvenile (2- to 3-month-oId), subadult (5-month-old), and adult (older than 7 months) individuals. Allometrical calculations were performed on the basis of body weight. Changes in size of different parts of the brain obtained to different degrees were found to be dependent on age but independent of body size. From the juvenile stage to the subadult stage, total brain size remains unchanged, although most major brain parts increase in size, while the grey matter of the isocortex decreases. During subsequent development from subadult to adult, total brain size evidently decreases. Within the brain all major structures also decrease in size, except for the medulla oblongata and the mesencephalon, which remains relatively stable in size. The grey matter of the isocortex shows the greatest decrease, followed by the allocortex and corpus striatum, the cerebellum, the white matter of the isocortex, and the diencepha-Ion. Thus, an unusual but evident 'overshoot' in size of the total brain and certain parts apparently occurs in this species before adulthood is reached. This phenomenon is discussed in connection with size changes concomitant with domestication as well as with cageing of individuals and with postnatal and seasonal size changes known from some soricid species as the so-called Dehnel phenomenon.