Changes in serum TSH, PRL, GH, T3, T4 and FT4-Index after injection of TRH (500 micrograms i.v.) were measured in 13 normal women and 12 normal men. In both sexes, TSH and PRL concentrations rose significantly over baseline after TRH injection (p less than 0.01). The PRL response, but not the TSH response, was significantly lower in men than in women. Although most TSH and PRL peaks occurred about 30 min after TRH injection, a delayed PRL peak was seen in four men. Thyroid hormones also rose significantly after TRH, with T3 peaking earlier (60 min) than T4 and FT4-Index (greater than or equal to 180 min). The TSH area under the curve correlated well with the TSH peak value (p less than 0.001), with the TSH value at 30 min (p less than 0.001), with the delta max TSH (p less than 0.001), and with the delta TSH measured at 30 min (delta 30 TSH) (p less than 0.001). The PRL area under the curve correlated well with the PRL peak value (p less than 0.005) and with delta max PRL (p less than 0.05) in both sexes, but the PRL value at 30 min was significantly correlated with the area under the curve only in women (p less than 0.005). These data indicate that in both sexes a baseline sample and a 30-min sample provide an excellent index of the entire TSH secretory response to TRH; the PRL response to TRH is clearly related to sex; and the 30-min value of PRL in women, but not in men, is an adequate measure of the PRL secretory response to TRH.