The presence of grayanotoxins (GTX-I, GTX-II and GTX-III) in honey samples from the Mediterranean Sea and Black Sea regions of Turkey and the effect of honey containing GTX-I on the threshold stimulus voltage of frog gastrocnemius muscle were studied. We used 2 groups of gastrocnemius muscles isolated from curarized or noncurarized frogs. The threshold voltage was measured by external perfusion with a polygraphic system. Isolated muscle preparations were perfused separately with Clark-frog-Ringer's solution containing honey solutions from the Mediterranean Sea region (normal) or with honey solutions from the Black Sea region (GTX-I-containing, poisonous). There was no significant difference between the normal and poisonous honey groups data with the curarized preparations. But in the noncurarized preparations, the honey containing GTX-I significantly decreased the threshold voltage in comparison with normal honey. These findings suggest the effect of GTX-I on frog neuromuscular junctions is due to an increase in membrane permeability to sodium ions.