Organophosphorus (OP) pesticide toxicity is a global health problem. However, the underlying mechanism is still unclear. OP pesticides are inhibitors of cholinesterases. Hypothesized mechanisms for respiratory failure include overstimulation of nicotinic acetylcholine receptors (nAChRs) and down regulation of post synaptic nAChRs, (De Bleecker,1995, Senanayake and Karalliedde,1987). We are addressing these hypotheses by studying OP toxicity in vivo and in vitro. Mice anesthetized with isoflurane and medical oxygen (0.4L/min) were treated with either saline placebo or the commercial OP pesticide dimethoate EC40 (emulsifiable concentrate;0.12mg-0.25mg/kg IP) and mechanically ventilated throughout the experiment. Noradrenaline (4mg%-20mg%) was used to maintain the mean blood pressure above 75mmHg. Mechanical muscle twitch strength was measured in response to indirect stimulation via the tibial nerve (train-of-four stimuli). Animals were maintained for 4-5hrs and showed a decline in neuromuscular function that was consistent with failure of neuromuscular transmission. To test the direct effect of OP compound in NMJ transmission, we made intracellular electrophysiological recordings of resting membrane potential, spontaneous miniature endplate potentials (MEPPs) and evoked endplate potentials (EPPs) in isolated sciatic nerve/flexor digitorum brevis muscles using different treatments in vitro. Experiments were conducted in Hepes buffered mammalian physiological saline (MPS) and action potentials were abolished with µ-conotoxin (2µM). We compared the effects of toxic minipig plasma (extracted in-house from minipigs treated with (orally) either dimethoate EC40, or non-formulated dimethoate active ingredient (AI) alone) with neostigmine as positive control. Toxic plasma abolished synaptic transmission within 30-60 minutes of bath application with little change in the time course of MEPPs or EPPs. By contrast, an increased half decay time was detected in neostigmine and dimethoate AI treated preparations, in both EPPs (Hepes MPS,3.44 ±0.60sec; control plasma,3.24±0.49sec; EC plasma,4.10±2.38sec; AI plasma,6.91±2.04sec; neostigmine,7.74±2.16sec: p < 0.001, ANOVA, n=20-30 fibres in each of 2-4 mice) and MEPPs (Hepes MPS,2.56±0.76sec; Ec plasma,2.81±0.67sec; AI plasma,4.55±0.90sec; neostigmine,3.66±0.81sec: p<0.001 ANOVA). However, there was no significant change in half-decay time in dimethoate EC plasma compared to control, indicating involvement of complex pathway in transmission failure. Furthermore, dimethoate EC plasma produced increased spontaneous activity compared to other treatments (MEPP frequency: Hepes MPS,0.86±0.45sec-1; EC plasma,3.04±0.91sec-1; AI plasma,0.63±0.27sec-1; neostigmine,0.66±0.44sec-1: p<0.001 one way ANOVA). The data indicate that neuromuscular transmission failure with commercially available OP pesticides cannot be explained solely by inhibition of acetylcholinesterase.