Hypokinesia (reduced motor activity) induces significant tissue morphological changes, however, it is not known if hypokinesia (HK) could contribute to morphological changes in the kidney. Measuring the nuclear size in the inner medullary collecting ducts on the kidney of rats the aim of this investigation was to disclose kidney morphological changes during prolonged HK.Studies were done on 192 13-week-old male rats (370 to 390 g) during 15 days pre-HK period and 90 days HK period. Rats were equally divided into 2 groups: vivarium control rats (VCR) and hypokinetic rats (HKR). All HKR were kept in small individual cages.A significant (p0.05) in VCR when compared with their pre-HK values.It was concluded that the nuclear size in the inner medullary collecting ducts of the kidney increases significantly during prolonged HK. The reason for this reaction remains unclear.