The purpose of this thesis was to investigate the rheological properties of non-dilute fibre suspensions. Glass fibres with aspect ratios of 56 and 95 were suspended in two viscous, Newtonian oils at two concentrations. The shear viscosity and first normal stress difference of the suspensions were measured. The shear viscosity was found to increase with suspending fluid viscosity, solid volume fraction and fibre aspect ratio. Shear thinning was observed. The first normal stress difference was found to vary linearly with shear rate and was found to increase with suspending fluid viscosity.Experiments in extension investigated the effect of the addition of fibres on breaking length, that is, on the maximum length to which a small sample of the suspension can be stretched into a filament without breaking. Breaking lengths of the suspensions were measured using a filament stretching rheometer and compared to the breaking lengths of their Newtonian base fluids. The addition of fibres was found to only slightly increase the breaking length at low extensional rates, and to decrease the breaking length at high extensional rates. The breaking length was observed to increase with decreasing fibre aspect ratio.