Oxytocin was injected once daily into eighteen Hereford heifers over a period corresponding to several oestrous cycle lengths. Oxytocin was injected at two dose rates (6 or 18 USP units/100 lb liveweight) twice in the day (a.m. or p.m.). Pretreatment cycle lengths were recorded to serve as control cycles. Mean oestrous cycle lengths of 19·4, 15·2 and 11·2 days, respectively, (P<0·001) were recorded in the controls and the groups receiving 6 and 18 units oxytocin. Heifers injected a.m. and p.m. had cycle lengths of 14·8 and 15·7 days, respectively (P>0·05).
Oxytocin produced epithelial cysts in the fimbria of the infundibulum, in the ampulla of the oviduct and in the mucosa of the endometrium of the uterus. There was mild cystic endometrial hyperplasia of the uterus, and occasional cyst formation in the rete ovarii. The epoophoron was dilated and distinct in many ovaries. Large luteinized follicles, apparently cystic, were present at slaughter in four heifers.