1. The structural integrities of various preparations of rat small intestine for the study of absorption in vitro have been compared after incubation or perfusion. 2. Perfused intestines removed from anaesthetized rats, and thus never deprived of a supply of oxygen, maintain their structural integrity even after perfusion for 1 h provided that a Krebs–Henseleit bicarbonate perfusate is used. However, intestines removed from freshly killed rats show severe villus disruption and oedema after perfusion for only 20 min. 3. Extensive damage to both crypts and villi is observed in everted sacs of small intestine incubated for 20 min, regardless of the buffer system used. Intestinal rings show damage at the tips of the villi after incubation for 2 min, but otherwise remain morphologically intact; this damage is progressive with time. 4. It is concluded that the exact mode of preparation of intestinal tissue is critical for preservation of structural and functional integrity and that this is especially important in quantitative studies on transport processes. Further, it is recommended that routine monitoring of the integrity of intestinal preparations in vitro is desirable and that histological assessment is an appropriate technique.

1. Water absorption rates by small intestine in vitro and the incidence of diarrhoea in vivo were estimated after 5-fluorouracil was injected into rats at various times of day and night. 2. Impairment of water absorption and the incidence of diarrhoea were minimal in rats on a normal dietary regimen when the 5-fluorouracil was injected at around 02.00 hours. 3. the DNA content of the mucosa of the small intestine from uninjected animals also showed a diurnal variation, with a maximum at around 03.00 hours. 4. Manipulation of the time of injections (possibly in relation to feeding time) provides a potential means for increasing the therapeutic value of 5-fluorouracil.

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.

1. Hydrolase activities against three dipeptides were measured in mucosal cytoplasm in unperfused intestines and in mucosal cytoplasm, luminal effluents and serosal secretions after perfusion in vitro and in vivo for 1 h. Intestines in vitro were prepared both from anaesthetized rats and from freshly killed rats. 2. Only 0·6–1·9% of the initial cytoplasmic activity was recovered in the luminal effluent when intestines in vitro were prepared from anaesthetized rats. Recoveries in luminal effluents were similar (1·3–3·3%) during perfusion in vivo. 3. Losses of dipeptidases into the luminal effluent were four to eight times greater when intestines in vitro were prepared from freshly killed animals. 4. Similar losses of dipeptidases into the secretion on to the serosal surface were observed; they too were much greater when intestines were prepared from freshly killed animals. 5. Small losses of mucosal DNA during perfusion were also observed; however, losses of cytoplasmic peptidases were consistently slightly greater. 6. Enzyme loss therefore probably occurs both by sloughing of whole cells and by a more specific process which is greatly influenced by experimental procedure. Caution is necessary in the interpretation of peptide transport experiments in vitro , although the possibility that intraluminal hydrolysis is of physiological significance must not be excluded.