The female carrier of the jimpy (jp) gene is a model system to study the plasticity of neuroglial cells and the mechanisms they use to compensate for a temporary deficit in myelin. Myelin in the female carriers is reduced 30-40% during the first postnatal month but is normal in adults. We hypothesized that the number of oligodendrocytes (OLs) in the female carriers is increased, based upon previous data showing OL proliferation is increased but the number of dying OLs is only slightly elevated in development. To test this hypothesis, antibodies to carbonic anhydrase (CA)II, an OL-specific marker, were used to quantify the number of OLs in the spinal cords of 1-month-old and adult female carriers. Contrary to expectations, the number of OLs is significantly reduced in the dorsal funiculus and grey matter by 21% in adult female carriers compared to controls. A reduction of lesser magnitude is present in the 1-month-old animals. Electron microscopic montages prepared from normal and carrier dorsal funiculus were used to count total numbers of glia. Ultrastructural quantification shows a similar reduction in the number of OLs and confirms the validity of the CAII immunostaining as a means to quantify OLs. These data show that there are 21% fewer OLs in the central nervous system (CNS) of adult female carriers but normal amounts of myelin. Presumably, some OLs in the carrier CNS are maintaining more myelin than their counterparts in normal CNS would. These findings demonstrate that (1) a reduction in number of OLs does not necessarily involve a reduction in the amount of myelin, and (2) OLs have considerable flexibility in the amount of myelin they can make.