Kinetic and concentration-mediated dependences of urea effects on toxicity and upon values of negative extremes (279, 287, 292 nm), shifts of lambda max fluorescence from 332 nm to the red region, decrease of fluorescence intensity, changes in accessibility of SH-groups were found. These data point to direct correlation between conformational changes in neurotoxin in urea solutions and decrease of toxicity. An investigation was carried out on renaturation of conformational changed neurotoxin in 8 M urea solution. In no cases was the toxicity restored, though there was a shift of lambda max fluorescence in protein from 340 to 333 nm, that was the evidence for reverse transition of the neurotoxin molecule from a disordered structure to the globule shape. The data lead to the conclusion that the toxicity region is probably a conformationally depending determinant.