Ovogenesis of the Barents sea sponge Leucosolenia complicata Mont. having asconoid canal system was studied at light (using histochemical methods) and ultrastructural levels. It has been established that the reproduction period "arranges" for autumn-winter season. The ovogenesis lasts for approximately two months. Grand accroissement is the longest stage of ovogenesis, lastiing in L. complicata from the end of Sptember to the middle of November. At this stage ovocytes lose their amebic movement, which coincides with the beginning of nurse cell formation. The nurse cells are derived from choanocytes. The choanocytes contacting with the ovocyte surface lose the collar and flagellum, grow in size and transform into nurse cells. Any cell of nurse-cell complex is capable of seizing a spermium and being transformed in situ into a carrier-cell. The specific role of nurse-cell complex is to synthesize sudanophilic (lipid) granules for the carrier cell and ovocyte. In the whole, a weak phagocytic activity of ovocytes, is characteristic of the ovogenesis of L. complicata. During vitellogenesis the ovocyte synthesises endogenic mucopolysaccharide granules from oligosaccharides originated from the mesohyl. Lipid granules pass to the ovocyte from nurse cells. Ribosomal RNAs are accumulated as a result of the own activity of nucleolus-nucleus apparatus of the ovocyte.