The Casiragua, Proechymis guairae is a South American rodent of the sub-order Hystricomorpha. In common with some other members of the sub-order the Casiragua has a long gestation period (62 days) relative to its size and it has been suggested that the Casiragua may be of value in biomedical research. We have, therefore, examined the reproductive physiology of this species in order to determine the optimum conditions for breeding experimental colonies and to establish a fund of basic data as an essential preliminary to more detailed studies. It emerges that the Casiragua is an induced ovulator (and not strictly a spontaneous ovulator) and shows no regular oestrous cycle when not pregnant. Furthermore, mature animals caged together for the first time tend to be very aggressive, inflicting serious, sometimes fatal, wounds on each other. We therefore conclude that, at present, the Casiragua does not appear to be a suitable species for routine laboratory use but its unusual reproductive characteristics make it of potential interest to endocrinologists and behaviouralists.