The locomotor activity patterns of the Drosophila melanogaster brain mutants optomotor blind (omb), lobula plateless (lop), minibrain (mnb), small optic lobes (sol), sine oculis (so), and the double mutants mnb;so and sol;so, all of which show reductions in the optic lobes, were investigated and compared with those of the wild-type. In none of the mutants was the number of arrhythmic flies significantly higher than in the wild-type, indicating that the optic lobes are not the sole site of a pacemaker controlling the locomotor activity rhythm. However, these mutations greatly influence the stability of the circadian system, in that the number of flies simultaneously showing two or more circadian components increased as the optic lobe defects became more severe. In flies with the strongest reduction of the optic lobes, two free-running circadian components were found almost exclusively. This suggests a two-oscillator control of the locomotor activity. Eyeless mutants also expressing a neural mutation were entrained by light:dark (LD) cycles, but their activity pattern in LD was changed compared to the wild-type and the eyeless mutant so.