Abstract: A structural element of a toy construction kit comprises a hollow structural member having an inner passage, formations for connecting this structural member with another structural member, and a cover member detachably engageable in the inner passage of the structural member and provided with formations forming an aperture, such as a window or a door. The cover member has at least a pair of slots extending from one of its ends toward another end and forming therebetween a section which is separated by the slots from a remainder section of this portion. The thus-formed section has an outer surface provided with a projection. The slots may extend up to the other end of the portion of the cover member.