A method for growing Azotobacter vinelandii under conditions of optimal aeration has been devised such that the culture could be sampled frequently so allowing growth and metabolism to be followed continuously. At medium culture densities a discontinuity was observed in the culture properties of both strains used for comparison throughout this work. This discontinuity was attributed to oxygen starvation of the cells and the different properties of the exponentially-growing cultures before and after this change have been noted. The intra- and extra-cellular pools of nitrogen compounds were examined. The presence of 3,4 dihydropyridazinone-5-carboxylic acid could not be confirmed but paper chromatography on its own was clearly inadequate to settle the status of hydrazine derivatives in fixation. The effect of artificial extracellular nitrogen pools on growth aml fixation ras examined using cultures in various states of training with respect to the substrate. The results were confirmed and expended using isotopically labelled ammonium and gaseous nitrogen. Fixation and ammonium uptake occur simultaneously in trained cultures: the implications cf this are discussed rind it is proposed that free ammonium can no longer be regarded as an obligatory intermediate in the fixation process. Urea is metabolised by the same routes as ammonium though its hydrolysis is not effected by a simple exogenous urease. Though L-asparagine and, to a lesser extent, L-glutamine are used Pr partial nitrogen sources, these effects are of minor importance. The amides, in common with L-aspartic, acid, L-glutamic acid and L-alanine, stimulate fixation (per unit cellular material synthesised) by quantities of up to 7O of its normal value. The excess nitrogen thus fixed is excreted either as peptides or in the form of more of the added amino acid itself. These results are Interpreted in terms of specific transport mechanisms and interference with structural peptide synthesis, which is assumed to take place in the cell envelope. The results are fitted into an overall scheme proposed to explain the fate of gaseous nitrogen in the metabolism of Azotobacter vinelandii.