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Plasma levels of LH were measured in young sexually mature (2- to 5-month-old) and aged (13- to 20-month-old) female C57BL/6 mice, Syrian hamsters and Wistar rats using a radioimmunoassay (RIA) and a radioreceptor assay (RRA). There were no statistically significant differences when comparing data from the two assays when examining young oestrous and dioestrous mice. Aged oestrous and dioestrous mice exhibited significantly higher levels of LH as measured by RIA than by RRA. Levels of LH analysed by RIA were also higher in aged mice compared with those in younger mice. Comparing LH concentrations with the two types of assays in younger dioestrous and aged anoestrous hamsters produced similar results. In contrast, aged pseudopregnant rats exhibited significantly lower levels of plasma LH than younger dioestrous females and there were no differences in RIA and RRA values. There were also no differences when comparing data from the two assays in younger rats. The mean (± S.E.M.) RRA: RIA ratios of 0·96 ± 0·05 (oestrous) and 0·96 ± 0·04 (dioestrous) for younger mice and 0·92 ± 0·08 for younger hamsters were significantly higher than the ratios of 0·66 ± 0·72± 0·05 and 0·71 ± 0·05 respectively of their aged counterparts. The mean RRA: RIA ratios in the two age groups of rats were almost identical (0·79 ± 0·05 and 0·80 ± 0·10). These studies suggested that a portion of the higher LH levels detectable by RIA in some aged female rodents results from qualitative changes in the LH molecule.