Seminar Abstract: Neurons communicate via the release of neurotransmitters at synapses. It has been generally assumed that neurons in the mammalian brain utilize a single fast acting neurotransmitter and release the same substance at all of its synapses. I will present data from our laboratory and others that demonstrate a much higher complexity to neurotransmission. In older brain regions, such as the basal ganglia, many neurons release multiple small molecule neurotransmitters, such as GABA, glutamate, dopamine and acetylcholine, often targeting different cells with different transmitters. We find that in different classes of neurons, the release of collections of neurotransmitters serves different purpose, in some circuits acting as a substrate for plasticity and in others triggering cascades of synaptic signaling that evolve broad time scales. I will conclude by speculating about the contributions of multitransmitter neurons to the function of mammalian cortex and basal ganglia during learning .