Cells of the gas gland of the perch Perca fluviatilis L., stimulated to increased generation of gas by the repeated emptying of the swim-bladder, were examined in the electron microscope. Intense activity of the nuclear envelope was demonstrated. Simple vesicles originating from the external nuclear membrane and the so-called multivesicular bodies derived from the outpocketings of both membranes of the nuclear envelope were observed. The multivesicular bodies were filled with numerous fine vesiculae arising from the active proliferation of their internal membrane. The authors offer two alternative mechanisms of formation of fine vesiculae inside the multivesicular bodies and the mechanism of the tearing away of these bodies from the nuclear envelope.