A sensitive and reproducible double antibody radio-immunoassay for rat LH measurement, which requires at most 50 μl of serum or plasma per tube, is described for use with ^l25I and rabbit anti-rat LH serum. The assay has a sensitivity of 16 ng LH/ml, and coefficients of inter-assay and intra-assay variations of ± 6,6% and ± 3.1% respectively. The possible diffusion of the constituents of the incubate through the walls of the plastic micro tube used, as described by others, was not significant. Normal basal LH values for albino rats are described together with their serum LH responses to various doses of LRH.