Between 1000 and 1200 hr on any given morning, female rats displaying regular 4-day estrous cycles were bled by decapitation. Plasma TSH concentrations were determined by homologous radioimmunoassay (RIA), in terms of NIAMDD-Rat-TSH-I-1 standards. Mean plasma TSH concentrations plus or minus SE were 1.2 plus or minus 0.1, 1.5 plus or minus 0.2, 2.3 plus or minus 0.6, and 1.1 plus or minus 0.1 ng/ml for rats killed at proestrus, estrus, diestrus-1, and diestrus-2, respectively. Statistical analysis revealed no significant differences among the TSH levels, except for the elevation at diestrus-1, which is of borderline significance at the 0.05 probability level. Our findings, based on RIA, are in contrast to previous reports, based on bioassays, of an elevation of plasma TSH at estrus.