The effect of thiophosphamide (single dose of 4 mg/kg) and hydrazine sulfate (single dose of 60 mg/kg) administration, begun 4-5 days after the introduction of solid sarcoma 37 and continued daily for 15 days, on sulfadimezine (50 mg/kg) acetylation was studied in mice. A direct correlation between tumor weight and the percentage of sulfadimezine acetylation was observed. Thiophosphamide inhibited tumor growth and the rise in acetylation activity, while hydrazine sulfate had no inhibiting effect on either index. In control mice, thiophosphamide increased acetylation activity, indicating that this drug alters the ratio of rapid and slow forms of N-acetyltransferase. One of the possible causes of the fall in CoA levels in tissues affected by malignant neoplasms may be the increase in N-acetyltransferase activity, leading to the increased use of CoA. Thiophosphamide inhibition of acetylation may facilitate the preservation of normal CoA levels.