The value and sign of the derivative partial-derivative R/partial-deri
vative T in HTSC materials in the normal state are found to depend on
the measurement frequency. Being of the metallic type at dc and low fr
equency, R(T) turns into the dependence of the semiconductive type and
then again metallic, as the frequency increases. The phenomenon is ob
served at 10(3) - 10(8) Hz on ceramic, thin film and single crystal sa
mples of different HTSC materials.