Serial radioimmunoassay determinations of serum beta hCG and methotrexate were compared in two patients with nonmetastatic gestational trophoblastic neoplasia (NMGTN) treated with Goldstein's modification of Bagshawe's intermediate-dose methotrexate-citrovorum factor rescue-treatment program. Pretreatment beta hCG levels (mIU/ml) ranged within the outer limits of the 10(3) log level. Following intravenous methotrexate, sharp serum peaks between 10(-6) and 10(-5) M were observed. Plasma disappearance was rapid with a 3 log drop noted within 24 hr to levels incapable of inhibiting DNA synthesis. beta hCG levels manifested a 1 to 1.5 log drop over the 8 days of chemotherapy and complete remission was noted within 5 to 6 weeks of the first dose of methotrexate. No significant clinical or laboratory toxicity was observed. Although cell culture studies show that 100% of cell death can be achieved with serum levels of 10(-5) M in methotrexate-resistant choriocarcinoma, similar data do not exist for previously untreated trophoblastic neoplastic cells. These preliminary observations suggest that serum methotrexate levels are important for establishing sensitivity levels in a heterogeneous population of trophoblastic cells in NMGTN and that the total dose of methotrexate may be safely preselected on the basis of the pretreatment beta hCG.