Defining any diagenetic alteration of 15N/14N ratios in organic matter in marine sediments is important in studies of past and present global nitrogen cycles. Previous field and laboratory studies indicate that isotope fractionation between released ammonium (NH3) and residual organic matter is likely, due to preferential breakage of bonds containing 14N during deamination of amino acids. We examined the isotope ratio of dissolved NH3 in pore waters and bulk N in sediments at site 1227 on the Peru Margin (ODP Leg 201). This core provided an opportunity to study isotope behavior at long spatial/temporal scales. Pore waters at this site are characterized by chloride profiles that increase with depth due to upward diffusion from an underlying brine (Kastner et al., 1990), dissolved NH3 that increases continuously to 23 mM at 144 mbsf, and sulfate depletion to zero in the upper 40 m. Dissolved NH3 shows a pronounced decrease in del 15N, from 8.5 per mil at 2.85 mbsf to about 5 per mil at 49 mbsf. These values are consistently 2-5 per mil heavier than bulk sedimentary N within this interval (3-6 per mil). From 49.95 mbsf to 144 mbsf, del 15N of NH3 remains constant at about 5.5 per mil, averaging 3-4 per mil lighter than bulk sedimentary N in this zone (about 9 per mil). The pattern of NH3 and del 15N is consistent with the two biogeochemically distinct zones at this site. The transition in the 15N of dissolved NH3 nearly coincides with the horizon of sulfate depletion and the appearance of abundant methane, and with the depth where the number of dividing cells reach a minimum (38 mbsf). Below the sulfate reduction zone, NH3 is dominated by nitrogen diffusing upward from the deep brine. Within the upper 40 m, bacterial activity appears to result in a net release of NH3 enriched in 15N. However, it is possible that differential diffusion of the two isotopes plays a role, and experiments are underway to investigate the importance of this effect. Competing release and uptake of NH3 by different bacteria may also play a role.