The expression of the synapsin I gene is neuron-specific and developmentally regulated. As a step toward characterizing the molecular mechanisms that are responsible for its transcriptional regulation in vivo, we have generated transgenic mice that carry the chloramphenicol acetyltransferase (CAT) receptor gene under the control of approximately 4,300 nucleotides of 5'-flanking sequence of the rat synapsin I gene. In four independent transgenic mouse lines, high level CAT expression is observed specifically in the brain and other neural tissues. Two of these lines also exhibit notable CAT expression in testis. The transgene is expressed at similar levels in many different regions of the central nervous system. Immunohistochemical staining detects the CAT marker protein in various cell populations of neuronal morphology within the brain and the spinal cord. Transgene expression is developmentally regulated in a way that correlates well with the expression of the endogenous synapsin I gene. Both follow a characteristic, biphasic postnatal time course with a maximum around day 20. We conclude that the DNA region investigated contains cis-regulatory elements sufficient to drive the expression of a reporter gene in a spatial and temporal pattern that resembles the expression of the endogenous synapsin I gene.