The analysis of faecal progesterone metabolites for non-invasive reproductive monitoring is an established technique used in an increasing number of laboratories. All data confirm that faecal progesterone metabolites reflect luteal and/or placental function, and that faecal values show a similar pattern to that of plasma progesterone, but have a species-specific lag time of 12 h to > 2 days Although biological data are comparable within a single species, it is currently difficult to compare values of faecal progesterone metabolites among laboratories. This is due to the use of different extraction methods and antibodies with different cross-reactivities. The extraction efficiency in several studies was tested using [H-3]-progesterone added to faeces: the recovery rates varied from 10% to > 90%, depending on the use of buffer or an increasing portion of methanol or ethanol in the extraction solvent. Progesterone is metabolised to several 5a- or 5 beta-reduced pregnanes (pregnanediones, mono-and dihydroxylated pregnanes) prior to its excretion into the faeces and unmetabolised progesterone is barely present, if at all. Faecal pregnanes either have a 20-oxo. a 20a-, or a 20 beta-hydroxyl group and several immunoreactive metabolites of each group are present in the faeces of all species as yet investigated. Hence, assay systems detecting different C20-groups can be employed for their determination, though antibodies against progesterone are most widely used. Recent studies have shown that antibodies raised against pregnane immunogens conjugated at the 3-position are superior to specific progesterone antibodies. These pregnane antibodies show considerable cross-reactivities and, therefore, concurrently detect several metabolites sharing a similar C20 group. For this reason the assays are termed group-specific, They can be generally applied to a wide range of species and their use, in combination with an extraction procedure using a solvent with a high percentage of alcohol, is recommended for the analysis of faecal progesterone metabolites.