Organophosphate induced delayed neuropathy (OPIDN) has been studied extensively but the mechanisms of toxicity remain unclear. It is generally accepted that the inhibition and ageing (dealkylation) of the B-esterase neuropathy target esterase (NTE) is integral to axonal loss. At present, the only way of detecting compounds that induce OPIDN is the hen test, an animal model. In this study, we preliminary validated hen embryo brain spheroids (HEBS) for the study of organophosphate (OP) toxicity. Hen brain spheroids have been characterised previously, although they have never been fully optimised for OP testing. We optimised the levels of acetylcholine esterase (AChE) and neuropathy target esterase by adapting the culture technique and using chemically defined media. Spheroid cultures were maintained for 35 days and viability and enzyme levels were monitored over this time. Levels of AChE and NTE in this system remained stable over the 35 day period. Using transmission electron microscopy, we have shown synaptogenisis within HEBS earlier than previously suggested in spheroid culture.These studies indicate that HEBS may be useful for the study of OP-induced toxicity and that the long-term stability of the cultures makes it an ideal candidate for studying OPIDN. (C) 2003 Elsevier Ireland Ltd. All rights reserved.