Mellomåsen houses the Nordland Psychiatric Hospital's department for central Nordland – Salten. It includes four ward units, an occupational therapy department, a physiotherapy department, an adult psychiatric outpatient clinic, a child and adolescent psychiatric outpatient clinic, and administration. A kindergarden and school are in a separate building, as well as a treatment home for youth.

Internally, there is a great variation in the experience of the spaces. There are corridors of varying widths, a narrow room may open up to a larger one, rooms on the top floor have pitched roofs, while others have flat ceilings. Internal color and use of materials also contribute to the variation.
The plan is from Byggekunst 8/1984 p. 413.