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We measure the nitrogen abundance in 5 Turn Off(TO) stars of the GlobularClusters NGC 6397 and NGC 6752, and compare the cluster abundances with thoseof field stars of comparable metallicity. We determine the nitrogen abundancefrom the band head system at 3360 \AA, using spectra of resolution R=45000obtained with the UVES spectrograph on the VLT. We apply the same methodpreviously used on field stars, to allow a direct comparison of the results.Nitrogen is found to have the same abundance in two of the NGC 6397 stars, inspite of a difference of one order of magnitude in oxygen abundance betweenthem. In a third star of NGC 6397 the value is slightly lower, but compatiblewith the other two, within the uncertainties. All the stars in NGC 6397 areN-rich with respect to field objects of similar metallicity. The two stars inNGC 6752 show a difference in nitrogen abundance by over one order ofmagnitude. The same stars differ in the abundances of other elements such asNa, O and Li, only by a factor 3-4. The behaviour of N is different in the twoclusters: no variation is observed NGC 6397, while a large variation isobserved in NGC 6752. This is consistent with a picture in which the stars inNGC 6752 have been formed by a mixture of ``pristine'' material and materialwhich has been processed by an early generation of stars, referred to as``polluters''. The N abundances here reported will help to constrain theproperties of the polluters. In the case of NGC 6397 a simple pollution historyis probably not viable, since the observed variations in O abundances are notaccompanied by corresponding variations in N or Li.

N in turn-off stars of NGC 6397 and NGC 6752