Regeneration of the synovial lining in the synovectomized rabbit knee was studied using the scanning electron microscope. The resected synovia regenerated considerably 3 weeks after synovectomy. However, 44 weeks following excision, their surface morphology was still very different from that of the normal tissue of intact animals. The regenerated synovia were characterized by three main features: the large number of various patterns, the many fields harboring fibrillation and the almost total lack of a bubble layer (the latter was formerly shown to be predominant on normal, intact synovia). The surface morphology of the non-operated (contralateral) knee differed greatly from that of normal synovia. The surface of sham-operated synovia was totally covered by the bubble layer. The appearance of vast fields harboring fibrillations indicated deficient ultrastructural regeneration. The altered surface morphology of the contralateral synovia was a novel finding. We wonder whether it would be appropriate to propose that the systemic reaction induced by synovectomy of the experimental knee initiated the synovial appearances recorded on the contralateral knee. The data reported here rule out the possibility of using the contralateral leg as an intact control.