Cockroach (Periplaneta americana) visual system (Mote 1990) is supposed to have evolved to function especially in low light intensity conditions. They show morphological and functional adaptations to dim environment; i.e. slow type dynamic characteristics with temporal corner frequency of about 10 Hz, green and UV sensitivity in different photoreceptors and large spatial pooling of signals in visual neuropiles. Our earlier study (Weckström et al. 1993) showed that the photoreceptors produce also spike-like potentials, one or a few on the rising phase of the graded potential response. These ‘spikelets’ are likely to be back-propagated action potentials originating from the axon. Since graded responses are attenuated in the thin and long axons, we suggest that the signals could be transferred in the axons mainly by action potentials. The very thin and distant axons are not amenable to stable intracellular recordings of high quality. Hence we constructed a compartmental model with Hodgkin-Huxley type channel kinetics for simulating the conduction of the graded responses and the action potentials in the photoreceptor. The model contains several compartments for both the soma and the axon. Each compartment has a passive leak conductance and an active potassium conductance. Action potential generation sites in the axon have also an active sodium conductance. Ion channel kinetic parameters were estimated from single electrode voltage clamp recordings. Membrane specific resistance and capacitance were estimated from the measured impedance functions, and intracellular resistivity was fitted so that the attenuation of the graded responses in the model corresponded to the results of recordings from the photoreceptor soma and early axon. Results from the model confirm that the graded responses attenuate strongly on their way from the soma to the axon terminal. The back-propagated action potentials attenuate even more, and the spike remnants in the soma match the experimental results. The spike initiation zone may be localised to the axon. The study thus provides further evidence that the cockroach photoreceptors may signal to higher visual centres in the form of action potentials.