Three samples were selected to represent the top, middle, and lower parts of the section exposed in the experimental Mohole cores (Guadalupe site). Concentrations of U and Pb and the isotopic compositions of were determined in these samples. It is believed that 2 types of Pb exist in the core material: that occurring in clay minerals; and that occurring in authigenic minerals and/or biogenic debris. The isotopic composition of the clay Pb did not change during the lapse of time between the deposition of the lower and upper samples, and it is thought that the same continental drainage supplied these clays continuously throughout this period. We believe that the isotopic composition of the authigenic and biogenic Pb (which originates from Pb dissolved in sea water) did change during this same period - being initially less radiogenic than the average Pb in the oceans, then becoming more radiogenic later times. This change may result from relatively non-radiogenic leads being derived from predominantly basalt-surfaced drainages during middle Miocene, and radiogenic leads being derived later, at the end of the Miocene, from drainage surfaces dominated by crystalline plutons or their sedimentary equivalents. There is no geochromometric utility associated with the isotopic differences among the leads.