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The temperature-dependent fluidity of myelin of frog and rat peripheral nerve (Nervus ischiadicus) was studied using the spin label technique. In frog nerve a phase change was detected at 38 °C. In rat nerve no sharp phase change could be established, and the lipid-depleted frog and rat nerve also showed no transition. From the spectral data, it was concluded that in frog and rat nerve the lipid-protein interactions are different, i.e. species dependent. Ca 2+ -depletion of frog nerve caused a loss of transition, while rat nerve remained unaffected. Thus it was indicated that, in frog nerve, Ca 2+ is involved in the phase change. In the total lipid extract of frog nerve a phase change centered at 32 °C occurred, while the total lipid extract of rat nerve again showed no transition. It is suggested that a connexion exists between our results and investigations on the temperature dependence of an axonal conduction block of nerve.