Considering the novel functions for both acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) in the developing nervous system (reviewed in Layer and Willbold, Prog. Histochem. Cytochem., 1995) a quantitative survey of the spatiotemporal developmental profiles of both AChE and BuChE activity in the neonatal rat brain would be extremely useful. To that end, we collected six brain regions at seven developmental time points, (postnatal day 1, 4, 7, 12, 17, 21, adult; n > or = 3) and measured AChE and BuChE activity using both biochemical and histological methods. These results indicated that the developmental pattern of AChE and BuChE activity varied with respect to brain region and age: (1) the ontogeny of either AChE or BuChE specific activity in one region was not necessarily indicative of the developmental pattern of the same cholinesterase in other regions; (2) the AChE developmental profile in a given region did not necessarily predict the BuChE developmental pattern for that same region. The data were also analyzed from a different perspective, i.e., the ratio of BuChE-AChE activity, in order to determine if BuChE activity preceded AChE activity during development as has been proposed for the chick nervous system (Layer, Proc. Natl. Acad. Sci. USA, 1983). Our analysis showed that, in general, the BuChE-AChE ratio decreased as the region matured, data which parallel the pattern of development of these esterases in the chick nervous system.