The drainage of oil under gravitational forces has been an important mechanism in the production of many oil fields. In order to extend the economic implementation of gas injection into more marginal fields, a reduction in the uncertainties associated with gravity drainage is required. This paper describes a series of three tertiary, nitrogen experiments which investigated the effect of permeability on gravity drainage. The experiments were conducted at low pressure using long, vertical, water-wet sandstone cores and decane in the presence of connate brine. The residual oil saturation following 62 days of nitrogen injection in a 0.37 mu m 2 core was 0.26, following 53 days of nitrogen injection in a 1.5 /mu m 2 core was 0.10 and following 63 days of nitrogen injection in a 2.0 /mu m 2 core was 0.10. The variation of the oil and brine saturations were determined as a function of space and time for each experiment using a radioactive tracer technique. This independent measurement of both oil and brine in situ saturations is a new development and enables core artefacts to be identified and relative permeabilities to be derived. Detailed analysis of the 2 mu m 2 experiment showed that the oil relative permeability was independent of position and was only a function of oil saturation. The relative permeabilities are characterized by a zero asymptotic residual oil saturation and a Corey exponent of approximately four, which is higher than the value of three proposed from theoretical models of film drainage. A numerical simulation of this experiment gave a good match to the production and in situ saturation data. Determination of oil and brine relative permeabilities under a flow regime are presented which are representative of gravity drainage during gas injection. The method outlined gives added confidence when assessing field development options.