Oral administration of manganese (Mn) to young rats results in morphologic changes in the growth plate histologically resembling rickets. Previous investigations have indicated that one important factor in the development of Mn rickets is phosphate depletion, because of the precipitation of insoluble manganese phosphate in the gut. In the present study, the morphologic and biochemical changes in rickets induced by phosphate (P) depletion per se as well as in florid and healing Mn rickets were investigated. Rachitic changes were induced in young rats by giving dietary Mn (2%) or by phosphate depletion (0.02% P) for 25 days. The rachitic changes of the proximal tibial growth plate were quantitated with the use of stereologic methods. In addition, the growth plates were dissected into one upper and one lower part and proteoglycans were separately extracted with 4 M guanidine hydrochloride and purified by ultracentrifugation. In rats given phosphate in more than equimolar amounts relative to Mn, the rachitic changes were abolished. When rats with florid Mn rickets were given standard rat food, containing 1% P, rapid healing ensued. Moreover, similar rachitic changes were obtained by phosphate depletion alone. Proteoglycans from the lowermost tissue portion of rachitic growth plates readily formed aggregates with hyaluronic acid and also contained larger chondroitin sulphate chains than controls. Upon healing, these changes tended to normalize. The data support the concept that phosphate depletion is crucial in the pathogenesis of Mn rickets. Also, it seems that the biochemical changes in different forms of rickets are of a similar kind, which further underlines the importance of proteoglycans in enchondral bone formation.