It is well-known that laminar flow of a liquid in a duct is predicted to choke if the viscosity of the liquid increases exponentially with increasing pressure. In other words, the pressure drop in the duct is predicted to become unbounded when the volumetric flow rate reaches a critical finite value. Choking is not observed in practice, however: the reason why is investigated here. It is shown that choking is always predicted to occur if the viscosity is independent of temperature or heat generation by viscous dissipation is neglected. If the viscosity decreases exponentially with increasing temperature and heat generation is not neglected, however, and if the temperature field is fully developed or if the flow is adiabatic, it is shown that choking is predicted not to occur.