The general goal of this project is to understand how time influences Pavlovian learning, the fundamental learning process in which humans and animals learn to associate signals with biologically significant events. The study of conditioning is important in part because it is deeply involved in the development (and treatment) of many mental health problems, including (but not limited to) drug dependence and anxiety disorders. Many of the proposed experiments, which will be conducted with laboratory rats, will distinguish between two different ideas about how Pavlovian learning works: A view that emphasizes the importance of learning "trials" (occasions on which the signal occurs either with or without the significant event) and another view that ignores trials and emphasizes the organism's perception of time between presentations of the significant event. These ideas and others will be contrasted in number of experiments. Many experiments will investigate the effects of trial spacing in acquisition, whereas others will investigate the effects of trial spacing during learning treatments (extinction and counter conditioning) designed to replace old learning with new learning. All will uncover conditions that "optimize" learning by making it strong and durable. Other experiments will compare the effects of the passage of time and contextual change, and others will investigate the psychological processes involved in discriminating different temporal intervals. The results will increase our understanding of the influence of time and memory on conditioning, and will help reveal the interrelationships between timing and associative learning. Many of the results will also increase our understanding of the causes of relapse after therapy and suggest ways to help prevent or minimize that relapse.