The relationships between the hormone levels in urine and plasma before semicastration, before radiotherapy, and after radiotherapy were studied in four patients with seminoma. Pretreatment levels of estrogens were elevated in two patients. The operation and radiotherapy had only a small effect on estrogen excretion. In all patients, low testosterone excretion was observed preoperatively, whereas, the plasma testosterone values were normal. Under treatment, the latter values also decreased slightly. Due to therapy, the dihydrotestosterone concentration in blood also fell. In patients with reappearing metastases, progesterone blood levels demonstrated a tendancy to increase. The gonadotrophin picture could not be characterized by the Pregnosticon test.