Determination of trace element absorption using enriched stable isotopic tracers and faecal monitoring has required 5-10 days collection of faecal output. In this study, the possibility of using rare earths as non-absorbable markers for measurement of the intestinal uptake of iron and zinc with single or reduced faecal collection has been investigated. Seven healthy subjects consumed a standard solution labelled with Fe-57 tracer and Sm marker, and a Farina meal labelled with Fe-58 and Zn-70 tracers, and Yb marker. Marker and tracer contents of faecal samples provided after consumption were determined by NAA and ICP-MS after ion-exchange separation. Intestinal uptake was calculated from faecal recoveries of markers and tracers for individual and composites of sequential samples. The uptakes obtained from the first two individual samples were not significantly different from those obtained from composites of the first 2-3 sequential outputs. It should therefore be possible to determine intestinal uptake with single or reduced faecal collection for iron and zinc. Rare earths offer a new opportunity to investigate absorption in unconfined situations. (C) 1999 Elsevier Science Inc.