It is known that bilayer membranes formed with a variety of phospholipids behave similarly to corresponding monolayers at lateral pressures in the range of 30-50 dyn/cm. Under this context, we analyze the thermodynamic fluctuations in the lateral density of phospholipid molecules in a monolayer, distinguishing between global and local fluctuations. For local fluctuations, the problem of finding the correlation area is solved by dividing the space into equal domains of areas A(0) = 2000A, where A(0) is the minimum area in which there is correlation between molecules in A and those in A(0). Then, the probability of a defect being produced in area A is w(0) = exp(-A/alpha), where a is the available area per molecule. From this, we calculate the conductance (G(BLM)) of a hypothetical bilayer membrane as the product G(BLM) = P(open)G(defect)Omega, where the three terms are, respectively, the probability of bilayer spanning defect, the conductance of the defect, and the number of membrane domains experiencing a defect. We found that G values are of the order of experimental ones when r = 9-11 Angstrom, what compares to the Nagle and Scott proposed value for bilayer holes. The relaxation time of defects was also estimated from Einstein’s formula. and using the coefficient of lipid lateral diffusion reported by Thompson and Huang, giving a time of the order of 20 x 10(-7), too short for experimental detection.