Previous studies have suggested that some of the antitumor activity of declopramide (3-chloroprocainamide) could be due to its metabolites. One metabolite has been identified as N-acetyl-declopramide (N-acetyl-3-chloroprocainamide). The aim of this study is to investigate the bioactivity of N-acetyl-declopramide and to compare it with its parent compound. The data have shown that N-acetyl-declopramide inhibited tumor cell growth in vitro in HL60 and K562 cells, and in vivo in scid mice xenografted with a human brain astrocytoma (T24), which was evaluated after oral doses of 20 and 40 mg/kg given at 0, 24 and 48 hr +/- a single im dose of cisplatin (7.5 mg/kg). The action was presumably by inducing DNA strand breaks and apoptosis. No acute toxic symptoms and no body weight loss were observed. N-acetyl-declopramide given orally or im gave a similar drug level in mouse serum 30 minutes after administration (p > 0.05). It had a greater antitumor activity in vitro in HL60 or K562 cells and a similar efficacy of inhibiting tumor growth in vivo, when compared with declopramide. These data provided an explanation for the primary result obtained in this study, i.e. declopramide administered orally at 40 mg/kg gave the same efficacy of inhibiting tumor growth as im injection although oral administration had a lower bioavailability due to the formulation of N-acetyl-declopramide. Based on these data, it was concluded that the antitumor properties of declopramide administered orally were not compromised by metabolism to N-acetyl-declopramide because the latter also has strong antitumor properties.