1. Rats injected intraperitoneally (i.p.) with the Tris salt of 5-fluorouracil (1.44 mmol/kg) showed severe toxic symptoms and a 15% mortality. No mortality and no obvious toxicity, apart from intestinal toxicity, resulted after injection of the sodium salt at equimolar dosage.

2. The plasma pharmacokinetics of the two salts of 5-fluorouracil (1.44 mmol/kg) were different after i.p. injection: higher plasma levels were achieved and were sustained for longer after the Tris salt than after the sodium salt.

3. No differences in plasma pharmacokinetics were observed after intra-arterial (i.a.) injection of the two salts of 5-fluorouracil (6.576 mmol/kg).

4. Thus there are fundamental (and potentially important) differences between the biological properties of the sodium and Tris salts of 5-fluorouracil. These cannot be explained at present.

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