A simple but new theoretical approach is used to analyse ex-ante the impact of tariff reduction. This methodology is based on the assumption of a constant price relation between each direct substitutable product. No elasticities are needed, but accurate import and domestic prices of the most sensitive and representative product of each tariff line are required. The present contribution forms a partial sector static simulation model that minimises the reduction of domestic production prices for the agricultural sector consecutive of the WTO market access negotiations of the Doha round. Results are shown on two levels whereas one of them provides rapidly a preliminary hierarchy of the sensitive products tariff lines and their optimal number. The second level provides a post-WTO maximal domestic price for aggregated products for which demand and supply elasticities are known. These maximal prices can be used as exogenous variable in dynamic models.