Monte Carlo simulations of liquid formamide and dimethylformamide, using the united-atom OPLS potential energy functions, have been undertaken. Basic thermodynamic properties and radial distribution functions are largely unaffected by the inclusion of long-range electrostatic. interactions in the form of a reaction field, whereas the dipole-dipole correlation function is observed to be very sensitive. Static dielectric constants of the liquids were calculated by measuring the polarization of the systems as a function of an applied electric field. The dielectric constant calculated for dimethylformamide is in good agreement with experiment, whereas the computed value for formamide is too low. It is likely that treatment of explicit polarization is needed for hydrogen-bonded liquids and related systems including proteins to correctly describe dielectric behavior. The dependence of the calculated dielectric constants on system size and reaction field dielectric constant was also examined.