We present a software package for the simulation of very large neuronal networks on parallel computers. The package can be run on any system with an implementation of the Message Passing Interface standard. We also present some example results for a simple neuronal model in networks of up to a quarter of a million neurons. The full software package as well as usage and installation guidelines can be found in [http://cs.usfca.edu/neurosys]. 2000 Elsevier Science B.V. All rights reserved.