1. The excretion of 2,4-dimethyl-6-sulphanilamidopyrimidine (sulphasomidine; Elkosin) and 4-methoxy-2-methyl-6-sulphanilamidopyrimidine (sulphamethomidine) given orally was examined in man, rhesus monkey, rabbit and rat. 2. About 70% of sulphasomidine (0·1g./kg.) is excreted mainly unchanged in the urine by these species in 24hr.; less than 15% of the dose is acetylated and there is no marked species difference in the fate of this drug. 3. Sulphamethomidine is excreted more slowly than sulphasomidine, and in the rat, rabbit and monkey the main metabolite is the N4-acetyl derivative. In man, only 20–30% of the dose is excreted in 24hr. and nearly 70% of this is sulphamethomidine N1-glucuronide, which is also excreted by the monkey but not by the rat or rabbit. There is therefore a marked species difference in the metabolism of sulphamethomidine. 4. Sulphamethomidine N1-glucuronide was synthesized and shown to be identical with the glucuronide isolated from monkey urine. 5. Sulphasomidine, sulphamethomidine and sulphadimethoxine (2,4-dimethoxy-6-sulphanilamidopyrimidine) were acetylated by rabbit or monkey liver homogenates. Although sulphasomidine is poorly acetylated in vivo, it is acetylated in vitro at rates comparable with those of the other two drugs. 6. The solubilities, partition coefficients and plasma-protein-binding of the drugs were measured. 7. The results are discussed.

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