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The effect of experimental procedures designed to modify an intracellular phosphoinositide signalling pathway, which may be instrumental in the photophobic response of the protozoan ciliate Blepharisma japonicum, has been investigated. To assess this issue, the latency time of the photophobic response and the cell photoresponsiveness have been assayed employing newly developed computerized videorecording and standard macro-photographic methods. Cell incubation with neomycin, heparin and Li+, drugs known to greatly impede phosphoinositide turnover, causes evident dose-dependent changes in cell photomotile behaviour. The strongest effect on photoresponses is exerted by neomycin, a potent inhibitor of polyphosphoinositide hydrolysis. The presence of micromolar concentrations of neomycin in the cell medium causes both prolongation of response latency and decrease of cell photoresponsiveness. Neomycin at higher concentrations (> 10 microM) abolishes the cell response to light at the highest applied intensity. A slightly lower inhibition of cell responsiveness to light stimulation and prolongation of response latency are observed in cells incubated in the presence of heparin, an inositol trisphosphate receptor antagonist. Lithium ions, widely known to deplete the intracellular phosphoinositide pathway intermediate, inositol trisphosphate, added to the cell medium at millimolar level, also cause a slowly developing inhibitory effect on cell photoresponses. Mastoparan, a specific G-protein activator, efficiently mimics the effect of light stimulation. In dark-adapted ciliates, it elicits ciliary reversal with the response latency typical for ciliary reversal during the photophobic response. Sustained treatment of Blepharisma cells with mastoparan also suppresses the photoresponsiveness, as in the case of cell adaptation to light during prolonged illumination. The mastoparan-induced responses can be eliminated by pretreatment of the cells with neomycin. Moreover, using antibodies raised against bovine transducin, a cross-reacting protein with an apparent molecular mass of about 55 kDa in the Blepharisma cortex fraction is detected on immunoblots. The obtained results indirectly suggest that the changes in internal inositol trisphosphate level, possibly elicited by G-protein-coupled phospholipase C, might play a role in the photophobic response of Blepharisma. However, further experiments are necessary to clarify the possible coupling between the G-protein and the putative phospholipase C.