Two trials were conducted during the years 1985 - 86 at the experimental farm of the Agronomy Institute of Pisa University, with the aim of singling out the combined effects of three treatments: A) two herbicide mixtures forweed control (active compounds: Trifluralin + Fluorochloridride for the first one, and Linuron + Alachlor for the second one); B) two sowing densities (5 and 8 plânts/sqare meter) and C) three levels of nitrogen fertilization (0, 100 and 2OO kg/ha N). This experiment was conducted in a sunflower crop grown in dry soil and typical coastal lands of Toscany. Moreover the trial was realized in three different sowing times for each year: early, normal and late, with a delay of 15 days among them. A good weed control was obtained with both mixtures employed, while no influence was revealed for the various levels of density and sowing times on weeds, whose proliferation was emphasized with the greatest nitrogen level. It seems that achene yield may have been increased by chemicals; in fact, with an increment of 1 q/ha of weedy dry matter, there was a decrement of about 0.14 q/ha in achene yield. The greatest density (8 p/sq.m) and the highest nitrogen level (200 kg/ha) determined the highest achene production, whcrcas the best sowing time in our environnrcnt was belween the end of March and the beginning ot April.