The 'armed' spider Phoneutria nigriventer is responsible for most human accidents involving spiders in Brazil. The effects of fraction Tx1 (PhTx1) from the venom of this spider were investigated by physiological and morphological methods using the mouse phrenic nerve-diaphragm preparation. PhTx1 (1 and 5 microg) did not affect the twitch tension of muscle fibers under indirect electrical stimulation. At this same concentration, PhTx1 also did not alter the miniature end-plate potential (mepp) frequency and amplitude, nor did it change the resting membrane potential 60 min after addition to the preparation. Light microscopy (LM) revealed that in muscles incubated with PhTx1 a number of fibers were morphologically altered, as evidenced by microvacuolization and myofibril hypercontraction and loss within 15 min after toxin administration. Transmission electron microscopy (TEM) showed sarcoplasmic reticulum swelling, disorganization of the sarcomeres and mitochondrial damage, and occasionally, sarcolemmal discontinuities with a persisting basal membrane. The intra-muscular fascicles of the phrenic nerve showed myelinated axons with vacuolated myelin sheaths as well as peri- and intra-axoplasmic vacuoles. The neuromuscular junction changes were variable, but were rarely severe. Thus, although PhTx1 did not depolarize or hyperpolarize the neuromuscular junction, it was nevertheless toxic to a restricted number of muscle fibers and nerve structures. The site of action of PhTx1 may involve the sarcolemma and axolemma as suggested by the morphological abnormalities which could reflect hydroelectrolytic disturbances.