Injection of S-antigen (arrestin) antibody into the brain of the blow fly Calliphora vicina, appeared to reduce sensitivity to the photic effects of a light:dark cycle (LD; entrainment) or continuous "bright" light (LL; arrhythmicity). In LD, a proportion of the injected flies evaded entrainment or showed delayed entrainment. In bright LL, flies continued with a free-running rhythm, which remained unchanged (as in continuous darkness) or lengthened (as in "dim" LL). These results focus attention on four groups of arrestin-positive neurons in the fly's brain as potential components of the photoreceptive system.