Calcite particles were exposed to Mn2+ in aqueous solution to allow adsorption. The calcite particles were then packed, and the NMR longitudinal relaxation time T-1 of water saturating the interparticle pores was measured. NMR surface relaxivity was then computed as 1/(T-1 . S-p/Y-p), where S-p/V-p is the ratio of surface area to pore volume. Adsorbed manganese increases the NMR surface relaxivity to approximately 2.4 X 10(-3) cm/s, roughly 50 times the surface relaxivity of very pure calcite. Flowing water with 4 mu M Mn2+ through a porous limestone plug decreased its T-1 by a factor of 6. Iron as adsorbate produced relatively small increases in surface relaxivity. These results suggest that manganese dominates the surface relaxivity of limestone rocks in situ, which is important in the interpretation of NMR borehole logs. Observations suggest that some adsorbed manganese eventually becomes buried by freshly precipitated calcite. In particular, the surface relaxivity increased with initial adsorption, but gradually decreased at long reaction times, even as the aqueous manganese concentration continued to decrease. NMR evidently senses only the manganese in the outermost atomic layer, and thus might prove useful in more detailed studies Of adsorption.