The phototactic response of the eucaryotic alga Chlamydomonas reinhardtii is a valuable model system for investigating the molecular basis of sensory perception and response induction. The entire sequence of events from interaction of the stimulus with the photoreceptor to the directed movement of the cells can be studied in a single-celled organism eliminating complexities associated with multicellular organisms. In preliminary studies we have shown that the antipsychotic drugs chlorpromazine and haloperidol cause a light intensity dependent reversal of phototaxis. At a moderately high light intensity, untreated cells swim toward the light; in the presence of chlorpromazine or haloperidol cells swim away from the light. At low light intensity the drugs have little effect on behavior. We will use behavioral, pharmacological, and biochemical approaches to establish the molecular basis for these effects. We hypothesize that a dopamine-stimulated adenylate cyclase system is involved in controlling the phototactic response in Chlamydomonas. To test this hypothesis we will use quantitative phototaxis and motility assays to study the relationship between liht intensity and the phototactic response both in the presence and absence of drugs. If Chlamydomonas has a dopaminergic control system involved in phototaxis, drugs with known effects on such systems should have predictable effects on phototaxis. We will, therefore, examine the effects of drugs such as dopamine, epinephrine, norepinephrine, apomorphine, fluphenazine, (plus)-butaclamol, and (-)-butaclamol. If the drug studies are promising, we will use chemical and biological assays to look for dopamine or other catecholamines in Chlamydomonas. Cyclic AMP plays a central role in dopaminergic systems. Using radioimmunoassay techniques we will measure cyclic AMP in cells adapted to the light and to the dark in the presence and absence of various drugs. Previously we isolated a group of phototaxis mutants with altered behavior and characterized them. In this study we will analyze the mutants with respect to possible alterations in dopamine or cyclic AMP and attempt to correlate the results with those in the wild type.