This work was aimed to study, under different levels of radiation intercepted by the plants during the seed filling stage, the relationship between yield and vascularization in three concentric positions of the capitulum. At the end of flowering, we applied shading (to reduce intercepted radiation) and thinning (to increase it) to three culture plots: a shaded plot, a thinned plot and a shaded and thinned plot. One additional untreated plot was used as control. We harvested heads at flowering and at physiological maturity. We delimited on them three positions: outer, middle and inner. Portions of each position were extracted from physiological maturity heads and their yield components were determined. The remaining heads were fixed in F.A.A., soaked in paraffin, and transversely cut at seed insertion to measure vascularization variables (phloem and sieve tubes area, number of tranverse and longitudinal bundles and sieve pores diameter). The head area unit was used as a base in all measurements. Shading reduced dry weight in the three positions. The middle position showed the highest yield and the inner showed the lowest in the four plots. The yield of the former was high because its lower individual seed weight (decreasing from periphery to center in all plots) was compensated by a higher number of filled seeds. However, average sieve pores radius was similar among positions, and phloem and sieve tubes areas were similar among positions and treatments, which could not account for the differences in yield per head area unit between positions. This enables us to conclude that this variation would not be produced by vascularization lack.