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The conduction mechanism in organic heterojunction solar cells (OHSCs) has been discussed based on the results of low frequency noise spectroscopy. We prepared OHSCs composed of poly-(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene), regioregular poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester. The devices prepared exhibited 1/f noise at the bias voltage range from +2 V (forward) to -2 V (reverse). It was found that there were three regimes in the noise spectral density Si(f) in the forward bias voltage region. In the ohmic regime, Si(f) increased in proportional to the square of current. In the trap filling regime, Si(f) was constant, while it increased again in the space charge limited current regime. In addition to this behavior, the noise spectra of OHSCs in the reverse bias voltage region exhibited the superposition of 1/f noise and the generation-recombination noise indicating the existence of carrier trap sites. These phenomena observed were thought to result from the breakdown and/or alleviation of the conduction path, which was caused by thermal stress of current flow.