1. The nervous activity in single afferent gastric vagal units was recorded electrophysiologically from halothane-anaesthetized sheep with spontaneous reticulo-ruminal movements present.2. Sixty-six afferent units innervating gastric mechanoreceptors were isolated from fifteen sheep. The receptors were located mainly in the medial walls of the reticulum and the cranial sac of the dorsal rumen, and also in the reticular groove, the reticulo-ruminal fold, the dorsal and ventral sacs of the rumen and the omasal canal.3. The mean conduction velocity (C.V.) for twenty-seven units was 12.4 +/- 1.0 m/sec (S.E.). For units with a pathway in the dorsal vagal trunk, the mean C.V. was 14.5 +/- 1.0 m/sec (S.E.) and for units with a pathway in the ventral vagal trunk the mean C.V. was 6.6 +/- 0.5 m/sec (S.E.).4. From the receptors a slowly adapting response was elicited by tangential lengthening. These were tension receptors in series with contractile elements, as they were excited by increased tensions developed both passively by inflation of the viscus and actively by muscular contractions.5. Receptors in the reticulum and the rumen appeared to be situated deep in the muscle layers, whereas those in the reticular groove structures seemed to be more superficial and gave the in series tension receptor response as well as a response to light pressure.6. A resting discharge in tension receptor units was usually absent at low levels of distension but appeared and increased as the level of distension was raised. Intermittency and fluctuations in the resting discharge were related to intrinsic local movement involving the receptive fields. Increasing distension enhanced the intrinsic movements.7. Even after the removal of the abomasum, reticular and ruminal (primary cycle) movements were evoked by distending the reticulum. It is possible that this manoeuvre enhanced intrinsic movements, which, in turn, caused an increased excitatory afferent input to the ;gastric centres' from in series reticular tension receptors.8. The enhanced afferent discharge from reticular tension receptors elicited by an isometrically recorded reticular contraction reflexly inhibited the subsequent (primary cycle) contraction of the rumen.9. Very few receptors were located in the caudal regions of the rumen whereas the cranial sac is richly supplied with tension receptors. The idea that the cranial sac may serve as the reflexogenic zone for secondary cycle movements of the rumen is discussed.