The effects of cluster environment on the chemical evolution of galaxies I. NGC 4303.

We present new spectrophotometric observations extending from 3610-9400 A of 12 giant H II regions located in the disc of NGC 4303. The following three line ratios exhibit definite gradients across the disc: the metallicity index R_23_, [N II]/[O II], and [N II]/[S II]. By assuming a set of abundance gradients as input, we have computed a sequence of photoionization models which simultaneously predicts all three ratios over the observed portion of the galaxy disc, permitting us to infer accurate abundance information for nitrogen, oxygen, and sulphur. All three of these elements exhibit negative gradients with slope values which are close to those measured in many other spirals.In addition, N/O increases, and S/0 decreases with an increase of O/H, respectively. The observed oxygen and sulphur gradients are explained well by a closed-box model of chemical evolution, while the gradient for nitrogen is steeper than simple model predictions. A comparison of R_23_ and O/H gradients derived in this paper with published values for two other Virgo spirals and five well observed field objects indicates that the cluster and field spirals are indistinguishable in terms of their abundance properties.