Spectrophotometric profiles representing the unfolding induced by guanidine on Bothrops moojeni myotoxins-I (MjTX-I) and II (MjTX-II), Bothrops jararacussu bothropstoxin-I (BthTX-I) and Bothrops pirajai piratoxin-I (PrTX-I) were obtained and compared with those obtained with bovine ribonuclease A (RNAse) and trypsin. The molar (ε1M) and percent (ε1%) extinction coefficients were determined for the four myotoxins as well as for RNAse and trypsin as reference parameters. These coefficients were then used throughout this work. The changes in free energy (ΔGD H20) corresponding to zero guanidine concentration and the guanidine concentrations (Δ1 / 2) able to convert 50% of the molecules from the native to the unfolded state were determined. The values of ΔGD H20 ranged from 4.42 (BthTX-I) to 8.02 (MjTX-I) kcal / mole, compared with 6.47 and 6.88 kcal / mole for trypsin and RNAse, respectively. The values for ΔGD H20 and Δ1 / 2 showed that BthTX-I is the least stable among the four myotoxins assayed, with a Δ1 / 2 close to that of RNAse, while MjTX-II is conformationally the most stable. Monitoring of the unfolding of RNAse and PrTX-I by a 0 to 6 M urea gradient PAGE revealed transitions from the native (N) to the unfolded (U) state with ΔGN-U of 0.22 and 0.41 kcal / mole, respectively. Sigmoidal curves showed welldefined two-stage transitions for both proteins. Abbreviation Used: PLA2, phospholipase A2; BthTX-I, Bothrops jararacussu bothropstoxin I, PrTX-I, Bothrops pirajai piratoxin I, MjTX-I and -II, Bothrops moojeni myotoxins I and II; GuHCl, guanidine hydrochloride; ATEE, N-acetyl-l-tyrosine ethyl ester; ATrEE, N-acetyl-l-tryptophan ethyl ester, RNAse, bovine ribonuclease A, RC-RNAse, reduced and carboxymethylated RNAse.