Addition of nickel stimulated growth and nitrogenase activity of Pseudomonas saccharophila under nitrogen-limited chemolithotrophic conditions, apparently because of a significant increase in expression of uptake hydrogenase activity. Inhibition of hydrogenase expression by 50 muM EDTA was relieved by nickel over a wide concentration range (1 to 200 muM). Co, Zn, Mn, and Cu stimulated expression of hydrogenase activity, but to a much lesser degree than nickel, and Fe, Mg, SeO(4), and SeO(3) did not increase expression. Nickel in individual combination with Mg, Fe, SeO(3), and SeO(4) resulted in activities that were essentially the same as that with nickel alone. Hydrogenase synthesis required the presence of nickel, and repression by O(2) was alleviated by increasing the concentration of added nickel. Cells placed under hydrogenase derepression conditions showed progressive incorporation of radioactive nickel to a much greater extent than did cells which were not derepressed.