The morphology of Schistosoma mansoni eggs in intestinal tissues (oograms), and egg hatching in faeces, were studied after parenteral administration of praziquantel (PZQ) to infected mice. PZQ was given parentally in doses of 60 mg kg-1 for one day, five days or 10 days. Eleven days after initiation of therapy, oograms from all groups receiving PZQ showed more dead eggs than controls; a dose response was also observed. Depression of faecal egg hatching occurred within 24 hours of PZQ administration. Our observations suggest that PZQ kills most S. mansoni eggs in host tissues when administered in higher doses than are routinely recommended for treatment of intestinal schistosomiasis mansoni. In order to reduce the lifespan of metabolically active eggs in sensitive tissues, prolonged courses of PZQ could be used when treating central nervous system schistosomiasis.