This report describes the extraction of synthetic TRH and its metabolic conversion in the perfused guinea pig placenta. These studies were performed to obtain an estimate of fractional fetal TRH losses through the placenta and to determine if some of these losses are due to TRH metabolism. The in situ guinea pig placenta was perfused through an umbilical artery for 90 min, and placental effluent fractions were collected at timed intervals from the umbilical vein. Experiments were performed in which the perfusion buffer contained 0.01, 1, and 10 micrograms/ml or no synthetic TRH. Synthetic TRH was always perfused in the presence of 3H2O. In experiments in which TRH was perfused, the perfusion reservoir contents and placental effluent fractions were counted for 3H, and TRH and deamido-TRH were determined by RIA. Similarly, cyclo(His-Pro) was measured when 10 micrograms/ml TRH were perfused. When no TRH was perfused, the perfusion reservoir and placental effluent contents were processed to determine their content of TRH immunoreactivity. When synthetic TRH was perfused, steady state TRH concentrations were achieved in placental effluent fractions by 20-30 min. The single pass extraction of TRH by the placenta was 11.4 +/- 2.6% (mean +/- SE) compared to 56.9 +/- 7.0% for 3H2O (P less than 0.001). No significant difference was detected regardless of whether 10, 1, or 0.01 micrograms/ml TRH were perfused. A portion of the TRH that perfused the placenta was converted to deamido-TRH at all concentrations of perfused TRH. No conversion of TRH to cyclo(His-Pro) was noted when the highest concentration (10 micrograms/ml) of TRH was perfused. The conversion of TRH to TRH-OH was 4.2 +/- 0.7% in a single pass. When the perfusion buffer was devoid of synthetic TRH, a small but significant increase in the content of TRH immunoreactivity was noted in the placental effluent compared to that in the perfusion reservoir. This was not large enough to affect calculations of the placental extraction of TRH. These studies, in addition to demonstrating that the placenta contains TRH deamidase activity, suggest that losses of fetal TRH through the placenta are not large. They do not support the current impression, based on the fetal TSH response to maternal TSH administration, that the placenta is freely permeable to TRH.