The hypothesis that stone circle size increased significantly with the arrival of the horse has been current for more than 20 yr, but only recently has it been possible to structure tests. Data from sites in Wyorning and Alberta are presented; in the Wyoming case, they seem to support the hypothesis, while in the Alberta case they do not Closer examination of Wyoming data reveals that ethnic replacement could account for observed differences. The Alberta data reveal that very large stone circles and, by inference, very large lipis, were in use in Middle Prehistoric times some 2500 yr ago. Thus, the association of large circles with protohistoric and historic times is rejected. The present paper also examines a selection ol the variables that may have influenced stone circle size and qensity of placed stones. Stone circles likely reflect a considerable variety of anchoring methods, and in some instances may not even have been displaced upon site abandonment. Stone placement density and the presence of cultural debris in the stone course could provide means for inference of extent of displacement.