Abstract
1. Intra-uterine pressure changes were followed by radio-telemetry in ten ewes carrying one lamb in each horn. One endo-radiosonde was implanted in each horn some 2-3 weeks before term and the pressure changes in the two horns were recorded simultaneously thereafter.2. The first isolated pressure waves which could be ascribed to myometrial activity were detected about 12 hr before delivery. Once pressure waves had become established, it was found that one horn developed waves which were about double the amplitude of those in the other horn and of greater duration. Thus one horn became dominant and the lamb occupying this horn was the first to be presented at the cervix. There appeared to be no difference between the two horns in the frequency of the waves.3. Digital examination of the os led to a phase of increased pressure waves which was considered to be due to oxytocin release.4. These findings are discussed and it is suggested that some local factor maintains the dominance of one horn and that this factor may be related to the local influence of the placenta.