A collimated whole-body counter was used to measure the retention and distribution of radioactivity along the longitudinal axis of the body at several times during the 24 hours after the intravenous injection of 50 microCi of Tc-99m-diphosphonates. Whole-body retention (WBR) was measured together with regional uptakes in the following four areas: head, chest, bladder, and legs using two structurally related Tc-99m-diphosphonate skeletal imaging agents: 1-hydroxyethylidene diphosphonate (HEDP) and methylene diphosphonate (MDP). The average 24 hour WBR values in young males, reflecting skeletal uptake of these tracers, were 17.7 +/- 2.2% (n = 20) and 31.0 +/- 2.4% (n = 3), respectively. A model of skeletal clearance was developed using the sum of two exponentials. In normal volunteers the initial rapid clearance phase of both tracers had a half-time of about 1 hour, whereas the slower second phase clearance had a half-time of 22 hours with HEDP and 44 hours with MDP. The WBR is usually calculated for the entire body only at 24 hours, but with the improved spatial resolution of a collimated whole-body counter, regional measurements could potentially be done over shorter periods (6-8 hours) in order to simplify the procedure.