Motile bacteria are attracted by certain chemical and repelled by others, a process known as chemotaxis. The chemicals are sensed by some twenty different chemoreceptors. These send their information to one of four different Methyl-accepting Chemotaxis Proteins, MCP's. In a process known as excitation, the MCP tells the flagella whether to rotate counterclockwise, for attractants, or clockwise, for repellents. Then excitation is shut off (even though the attractant or repellent may still be present) by a process known as adaptation: This consists of a change in the level of methylation of MCP. The mechanism of excitation involves at least phosphorylation. Work in this and many other laboratories aim to uncover this mechanism further. The objective of the present research proposal is to find out how bacteria can modify their chemotactic behavior. Another way of stating that is to say: Can bacteria "learn"? Three kinds of learning - known to occur in animals - will be studied: habituation, sensitization, and conditioned learning. If bacteria can do any of these, then mutants will be isolated that fail. Then by use of these mutants the biochemistry of these "learning" processes will be elucidated. In the first, exploratory phase, the methods used for measuring chemotaxis are the capillary assay, swarm plates, a computerized assay, and use of tethered cells. The mechanisms discovered here will hopefully have relevance to understanding neurobiology, behavior, and learning in higher organisms, including man. Further, diseases of the nervous system can probably be helped by research of this sort.