The effects of several metals on microbial methane, carbon dioxide, and sulfide production and microbial ATP were examined in sediments from Spartina alterniflora communities. Anaerobically homogenized sediments were amended with 1,000 ppm (ratio of weight of metal to dry weight of sediment) of various metals. Time courses in controls were similar for CH(4), H(2)S, and CO(2), with short initial lags (0 to 4 h) followed by periods of constant gas production (1 to 2 days) and declining rates thereafter. Comparisons were made between control and experimental assays with respect to initial rates of production (after lag) and overall production. Methane evolution was inhibited both initially and overall by CH(3)HgCl, HgS, and NaAsO(2). A period of initial inhibition was followed by a period of overall stimulation with Hg, Pb, Ni, Cd, and Cu, all as chlorides, and with ZnSO(4), K(2)CrO(4), and K(2)Cr(2)O(7). Production of CO(2) was generally less affected by the addition of metals. Inhibition was noted with NaAsO(2), CH(3)HgCl, and Na(2)MoO(4). Minor stimulation of CO(2) production occurred over the long term with chlorides of Hg, Pb, and Fe. Sulfate reduction was inhibited in the short term by all metals tested and over the long term by all but FeCl(2) and NiCl(2). Microbial biomass was decreased by FeCl(2), K(2)Cr(2)O(7), ZnSO(4), CdCl(2), and CuCl(2) but remained generally unaffected by PbCl(2), HgCl(2), and NiCl(2). Although the majority of metals produced an immediate inhibition of methanogenesis, for several metals this was only a transient phenomenon followed by an overall stimulation. The initial suppression of methanogenesis may be relieved by precipitation, complexation, or transformation of the metal (possibly by methylation), with the subsequent stimulation resulting from a sustained inhibition of competing organisms (e.g., sulfate-reducing bacteria). For several environmentally significant metals, severe metal pollution may substantially alter the flow of carbon in sediments.