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A general study of the influence of bars on the distribution of chemical composition across disks of spiral galaxies is presented. This work is based on abundance surveys, employing narrow-band interference filters, performed on a large number ($>$ 50) of HII regions within a small sample of barred galaxies. Relations between O/H gradients and some bar properties (axis ratio, age, star formation activity) are established. For example, the correlation between the slopes of the global O/H gradients and bar axis ratios is discussed. Also, following a recent study of the oxygen distribution in the barred spiral NGC 3359, it is shown that a break in the slope of the abundance gradient indicates that the bar is young, in agreement with the presence of intense star formation activity observed along the bar. We then used star forming activity along bars as an indicator of the age of the barred structure. From this, we suggest that the slope of the global O/H gradients in barred spirals becomes flatter when star formation becomes moderate or absent along the bar, and at the same time becomes active in the galaxy center. All these results are in good agreement with recent numerical simulations of barred galaxies, indicating that large-scale mixing due to gas flows induced by bars plays a major role in the chemical evolution of galaxies.\\ \indent This work was supported by NSERC (Canada) and FCAR (Qu\'ebec) postdoctoral fellowships, and in part by the NSF through grant AST 90-19150.
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