A unique high-pressure rheology apparatus is used to study the in situ formation and flow properties of gas hydrates from a water-in-crude oil emulsion. Viscosity and pressure of the hydrate slurry are measured during hydrate formation, growth, aggregation, and dissociation. The rheology of the hydrate slurries varies with time, shear rate (1-500 s(-1)), water content (0-50%), and temperature (0-6 degrees C). Hydrate slurry viscosity increases rapidly with time when hydrates form and then decays after going through a maximum as hydrate aggregates breakup or rearrange. Yield stress increases with annealing time up to 8 h and then remains constant. Hydrate slurry viscosity decreases with an increasing shear rate (i.e., they are shear thinning). Viscosity and yield stress both increase with an increasing water content. During dissociation, the viscosity increases just before the hydrate equilibrium temperature. Finally, transient viscosity measurements at varying temperatures suggest that mechanisms, such as cohesion forces and shear forces, competitively affect hydrate slurry viscosity.