Abstract 1. 1. Investigations on the neuromuscular transmission of a spider, Eurypelma hentzi Girard, were performed for the first time. The composition of an artificial saline solution is given. The membrane of the fibers of the musculus flexor metatarsi bilobatus (MFMB) behaves as a K +-electrode. Each fiber receives three motoraxons. 2. 2. Efferent stimulation of motoraxons produces small excitatory junction potentials (EJPs) of three types, which are very similar in height and rise time, nevertheless the contraction evoked is quite different. The EJPs show summation and facilitation; no inhibition was found. 3. 3. An increase in the external Ca 2+ concentration from 2 to 12 mM/l increases the amplitude of all junction potentials significantly; above that there is a decline in the height of the EJPs. The membrane potential is not affected. 4. 4. In high Ca 2+ concentrations the EJPs usually reach the critical depolarization (30 mV) for spike discharge. Action potentials vary in amplitude; sometimes there is a positive overshoot. They make the membrane partly inactive for a closely following second action potential, but not for a junction potential. 5. 5. The contraction is always augmented in higher Ca 2+ concentrations. Both action potentials and junction potentials can elicit twitches of the muscle with single shocks, the ones produced by action potentials are always much more powerful.