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A specially designed relaxometer was used to measure the stress relaxation under 30% elongation for tread rubber compounds based on a blend of three rubbers: natural rubber, neodymium butadiene rubber, and oil-extended styrene butadiene rubber. The rubber compounds were filled with carbon black and silica in different ratios: with 80 parts carbon black only, with 80 parts silica only, and with different ratios of carbon black and silica. The method proposed by Bartenev was used to calculate the relaxation specta. The maxima of the spectra at different relaxation times were assigned to different types of interaction: rubber- filler interaction, carbon black-carbon black interaction, silica-silica interaction, carbon black-silica interaction. New maxima were obtained for specimens containing roughly equal amounts of carbon black and silica. These results were compared with data obtained using transmission electron microscopy. The existence of three levels of structure of filled elastomers and the presence of 'rigid' rubber around filler particles are suggested. [ABSTRACT FROM AUTHOR]