The development of endolymphatic hydrops (e. hydrops) following secondary e. sac immune response was investigated in Hartley guinea pigs, for a period of 5 weeks. E. hydrops immediately developed to a maximum from day 2 to day 7 and then gradually reduced in the next 4 weeks. In the e. sac, numerous inflammatory cellular infiltrates, mainly polymorphonuclear cells and macrophages, were seen from day 1 to day 2. Lymphocytes and plasma cells appeared from day 3 and increased to a maximum by day 7 and then gradually decreased in the next 4 weeks. Neither primary e. sac KLH challenge nor e. sac PBS inoculation could derive e. hydrops. Development of e. hydrops was considerably parallel to the grade of immune reaction within the e. sac, suggesting that immuno-pathological reaction of the e. sac has an important effect on regulation of the endolymph volume.