1. To study the validity of mean whole-body intracellular hydrogen ion activity measurements ([H+]i) by the distribution of 14C-labelled 5,5-dimethyl-2,4-oxazo-lidinedione ([14C]DMO), eight anaesthetized dogs received three separate injections of iso-osmotic buffered chemical DMO, in cumulative doses of 50–2000 mg/kg body weight, following a control [H+]i determination by the 14C-tracer technique.

2. A correlation between dose of chemical DMO and measured [H+]i was found with a regression line of −Δ[H+]i = 23.27 + 16.45 [log DMO dose −2.41].

3. Although the results are consistent with active transport of DMO across cell membranes, other possible explanations such as altered cell buffering capacity by DMO loading and variable sequestration of DMO in the gastrointestinal tract must be considered.

4. There is no evidence for cellular binding of DMO.

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