Brain microdialysis allows neurotransmitters released in localized brain regions of live animals to be sampled. In the research described, microdialysis is used to measure the release of neurotransmitters that modulate networks of neurons at various levels of the nervous system. The neural systems that reward feeding behavior are studied. The results also apply to artificial rewards, such as electrical self-stimulation and addictive drugs. Positive feedback loops at various brain levels are outlined, and the possibility of a chemical code for different behavior patterns is presented. Results with microdialysis show that neurochemical release can be conditioned. Thus some aspects of learning are reflected in the neuromodulators which affect active components of neural networks.