The Norwegian government has developed a complex fiscal regime since 1963 to cover the activities of oil companies in the North Sea. The purpose of this article is to identify and quantify the financial effects of the fiscal system on operating companies involved in the Norwegian sector. The emphasis is on the effect of the tax package introduced by the government in the Odelsting Proposition No. 26 of February 1975. The impact of the tax is not found to be well related to the profitability of fields and the sensitivity of the tax burden to key variables such as reserves, oil price, capital and operating costs and inflation is examined. The most promising change appears to be a discriminatory refund or remission of royalty payments to marginal fields. The Act's main innovation, Special Tax, fails to discriminate between fields of profitability and could influence development decisions in marginal fields. The special allowance provision, by reducing the real cost of capital expenditure and favoring capital-intensive operations, could influence the techniques of exploitation. The effect of quasi-fiscal measures, particularly of government participation via Statoil, is also examined. 10 references.