1 Male albino mice were injected s.c. with an organopho sphate (mipafox, ecothiopate or paraoxon). Treat ments were either a single injection or multiple daily injections with lower doses for 5 or 8 days. At 3 h after injection the activity of brain and diaphragm acet ylcholinesterase and of brain neuropathy target esterase (NTE) was measured. Also measured in the diaphragm at 3 h post dose was the duration of spontaneous miniature endplate potentials (eMEPPs), recorded extracellularly.

2 At 7 and 28 days after dosing action potentials and evoked endplate potentials, produced by stimulating the phrenic nerve at 30 Hz, were recorded in diaphragm muscle. The amplitudes, time-course and latencies of these potentials were measured and the variability of latencies (jitter) was calculated.

3 Single doses of mipafox (20 mg/kg), ecothiopate (0.192 mg/kg) or paraoxon (0.415 mg/kg) in the mouse produced ca. 70% inhibition of diaphragm acetylcho linesterase at 3 h after dosing. All three OPs produced a prolongation of the half-decay times of eMEPPs.

4 All three OPs in the above single doses produced increased muscle action potential (postjunctional) jitter but only mipafox produced an increase in endplate potential (prejunctional) jitter. Mipafox in a slightly reduced single dose (17.5 mg/kg) had no effect on prejunctional or postjunctional jitter.

5 Multiple dosing with mipafox (8 mg/kg daily for 5 days) increased both postjunctional and prejunctional jitter at both 7 and 28 days after the end of dosing. After multiple dosing with mipafox (5 mg/kg daily for 5 days) postjunctional (but not prejunctional) jitter was increased. Multiple doses of paraoxon (0.166 mg/kg daily for 5 days) or ecothiopate (0.76 mg/kg daily for 5 days) increased prejunctional and postjunctional jitter.

6 Depending on the dosing regime, all three OPs tested were capable of increasing both prejunctional and postjunctional jitter. Neither ecothiopate nor paraox on inhibited NTE, so this prejunctional effect is not likely to be related to 'classical' OP-induced delayed neuropathy. The prejunctional effects may be related to long-term inhibition of acetylcholinesterase and the triggering mechanism for increase in prejunctional jitter may involve a relationship between the inhibi tion of acetylcholinesterase and the time for which it is inhibited. The differences between the time-courses of increases in prejunctional and postjunctional jitter and the differential effects of the different multiple dosing regimes indicate that it is likely that the triggering relationship between enzyme inhibition and time is different for prejunctional and postjunc tional effects.