When a reservoir is depleted, the lightest components (methane, ethane, etc.) can exsolve from the crude oil and create a gaseous phase. In conventional oils, the bubbles grow and coalesce quickly and the gas usually separates from the oil (slug flow). On the contrary, in heavy oils, bubbles are small, remain dispersed, and flow within the oil for a long time. This "foamy oil" phenomenon can drastically change the flow properties of the crude oil. This paper is devoted to the characterization of the heavy oil foaming behavior through a theological study. Our objectives are to study the kinetics of bubble evolution in heavy oil and to measure the influence of the bubbles on the heavy oil viscosity. A new experimental method was developed on the basis of rheological measurements under pressure. Several heavy oils containing dissolved gas have been depleted inside the pressure cells of controlled stress rheometers to create foamy oils. Viscosity and viscoelastic properties have been continuously measured using respectively continuous and oscillatory tests from the nucleation to the disengagement of bubbles from oil. Results reveal that, under low shear rates, the presence of bubbles leads to an increase in heavy oil viscosity, as predicted by the hard sphere model or Taylor. A theoretical model describing the viscosity of foamy oil was then established. It takes into account both first-order kinetics of the appearance and release of bubbles in oil and a classic suspension model. Good agreement was obtained between experimental data and model predictions. Several tests reveal the strong influence of the shear rate on the foamy oil behavior and point out the major role of bubble deformation on the viscosity of foamy oils, as shown previously in other viscous materials, such as magmas and polymers. Under high shear rates, we suggest that the stabilization of the elongated bubbles in oil leads to the establishment of an anisotropic material, which can be seen as a sandwich-like structure. As a result, the viscosity appears lower in the direction of the flow.