The mean-square electric dipole moment [mu(2)] was determined for 12 samples of atactic (a-) oligo- and poly(methyl methacrylate)s (PMMA), each with the fraction of racemic diads f(x) = 0.79, in the range of the weight-average degree of polymerization x(W) from 3 to 1.19 x 10(3) and for 11 samples of isotactic (i-) PMMA and its oligomers, each with f(r) similar or equal to 0.01, in the range of x(W) from 3 to 5.88 x 10(2), both in benzene at 30.0 degrees C. The determination was also made for a syndiotactic (a-) PMMA sample with f(r) = 0.92 and x(W) = 3.76 x 10(2) and for methyl isobutyrate in the same solvent condition. For both a-and i-PMMAs,the ratio [mu(2)]/x(W) as a function of x(W) decreases monotonically with increasing x(W) for x(W) similar or equal to 20 and then approaches its asymptotic value ([mu(2)]/x(W))(infinity), i.e., 1.92(7) and 2.12(6) D-2 for a- and i-PMMAs, respectively, indicating that the excluded-volume effect on [mu(2)] is negligibly small if any. From a comparision of these values of ([mu(2)]/x(W))(infinity) along with that of [mu(2)]/x(W) obtained for the s-PMMA sample, it is found that ([mu(2)]/x(W))(infinity) decreases with increasing f(r). An analysis of the present experimental values for a- and i-PMMAs is made by the use of the theory for the (unperturbed) helical wormlike chain modified so as to take into account possible effects of chain ends. Then it is shown that the above dependence of [mu(2)]/x(W) on x(W) may be explained by this modified theory with the use of the values of the model parameters already determined from an analysis of the mean-square radius of gyration, if the local electric dipole moment; vectors are properly assigned to the initiating and terminating repeat units. Unfortunately, however, any useful information about the main-chain conformations of a-and i-PMMAs cannot be obtained from an analysis of the dependence of [mu(2)] on x(W).