Microtubule protein, isolated from porcine brain by temperature-dependent assembly-disassembly cycles, was labelled with two types of nitroxide spin labels, maleimide and isothiocyanate. Labelling was performed either in depolymerized or polymerized form of the protein. Electron paramagnetic resonance spectroscopic measurements revealed 34 ns rotational correlation time of the labels in disassembled microtubule protein which corresponds most likely to the rotational motion of the subunits. Upon polymerization, changes in the rotational dynamics of microtubule protein occurred in the temperature range of 20-30 degrees C. Polymerization process was revealed as a transition between two states, one characterizing the tubulin in its monomeric form and the other the polymeric form. In the temperature range of 20-30 degrees C, both forms (monomer-polymer) of tubulin were observed. Very slow rotational motion in the millisecond time range was detected in microtubule pellet. Orientation dependence in the distribution of spin labels in macroscopically oriented microtubules was not found.