A one-dimensional transient two-fluid model is developed to predict transient slug flow in pipelines. To account for the interphase interactions, new constitutive relations for the drag coefficient and the virtual mass force for the slug flow regime are derived by applying the conservation equations to a geometrically simplified slug unit. New coefficients in the pressure gradient term in the two-fluid momentum conservation equations are also obtained to account for the non-uniform distribution of the phases and of the pressure drop along a slug unit. The new relations yield a more accurate treatment of the hydrodynamics of slug flow than traditional two-fluid models. Constitutive relations for other Bow regimes can also be incorporated into the model, allowing the analysis of general transient two-phase flows in pipelines.