Plasma progesterone concentrations were measured by radioimmunoassay (Law et al.,
1992). Assay sensitivity was 0.3 ng ml−1 and the intra- and inter-assay coefficients of variation
were 3.8 and 11.3%, respectively. Luteal tissue was prepared following extraction
methods adapted from Tsang et al. (1990). Supernatant derived from processed luteal tissue
was diluted and assayed as described for plasma. Assay sensitivity was 0.2 ng ml−1 and
the intra- and inter-assay coefficients of variation were 3.5 and 8.3%, respectively. Plasma
oestradiol concentrations were measured by radioimmunoassay