We have studied the critical micellar concentration (c.m.c.) of the homologs of the local anesthetic heptacaine in the medium 0.1 mol/l NaCl, employing spectrophotometry in the UV region of the spectrum at T = 21 degrees C and pH = 4.5-5.0. On the basis of our experimental results, it is assumed that in the homologous series of amphiphilic compounds the quasi-parabolic dependence of biological potencies on the length of their hydrophobic substituent ("cut-off" effect) could be caused by c.m.c.