In this study we investigated the mutualistic influence of the biologically active substances in the saliva of Hirudo verbana on 1) general conditions, 2) morphometric parameters of the body and immunological organs (thymus and spleen), and 3) hematologic and immunologic parameters (absorption and metabolic activity of neutrophils in the host organism’s blood) of outbred female rats and the offspring itself on the 1^st, 15^th, 30^th, 45^th and 60^th days of early postembryonic ontogeny. A hungry leech with a mass of 400 mg was attached to the interscapular area of the female rats once a week, 2 weeks before and 2 weeks after mating with males of the same age. The modulating effect of the biologically active substances in the ectoparasite saliva on the rat organism, the stimulation of the myeloid and lymphoid tissues and the increase in the functional activity of the neutrophils in the host’s blood was revealed for the first time.