We have recently reported an increase in the density of muscarinic cholinergic receptors in mice neonatally exposed to a persistent environmental agent, dichlorodiphenyltrichloroethane (DDT), and a subsequent exposure as adults to nonpersistent toxicants, such as bioallethrin or paraoxon. Here we have examined the effects of an exposure like this on muscarinic receptor mRNA expression. Ten-day-old Naval Medical Research Institute mice received a single oral dose of DDT (0.5 mg/kg body weight). When aged 5 months, they received bioallethrin (0.7 mg/kg body weight per day for 7 days) or paraoxon (1.4 mg/kg body weight every second day for 7 days). mRNA expression of subtypes m1, m3, and m4 was studied in 7-month-old animals. Changes could only be discovered in the DDT-bioallethrin treated mice, where expression of subtype m4 was elevated in cortex and caudate putamen. Moreover, the expression pattern of the subtypes m1, m3, and m4 in mouse brains was found to be very similar to that seen in rats, except for slight differences in the pyramidal cell layer of the hippocampus, where the outermost part of the CA3 region did not show any m4 hybridization. The present study indicates that the earlier observed increase in muscarinic receptor density in mice exposed as neonates to DDT and as adults to bioallethrin can be attributed to changes in the expression of m4.