Breath H2 and CH4 daily excretion patterns were studied in 20 healthy individuals whilst on their regular diets and fasting. The average H2 excretion level was found to be 85 ml day-' on habitual meals and less than 35 ml day-1 whilst fasting. The mean excretion in breath at any one time was less than 0-5 jimol I1 and individual breath sample excretion rarely exceeded 090 jumol .1-1. H2 excretion followed a regular pattern, being high in the morning, falling until about mid-day and rising during the early afternoon. Anaerobic bacteria in the gastrointestinal tract of man produce partially degraded carbon compounds and a number of gases during metabolism of carbohydrates. Of these the major ones are volatile fatty acids (VFA) and the gases carbon dioxide (CO2), hydrogen (H2) and methane (CH4). The formation of H2 and CH4 is unique to anaerobic bacteria and no higher animal cell is known to produce these gases [Mangold, 1934].In healthy adult man, the whole gastrointestinal tract harbours some commensal bacteria; but significant numbers occur only in caecum and colon.More than 95% of colonic bacteria are anaerobic [Hill and Drasar, 1975].Thus, in the normal situation, H2 and CH4 would be expected to be formed in significant amount only in the caecum and colon. Once formed the two gases *WHO fellow. 85

Multiple applied current impedance measurement systems require numbers of current sources which operate simultaneously at the same frequency and within the same phase but at variable amplitudes. Investigations into the performance of some integrated operational transconductance amplifiers as variable current sources are described. Measurements of breakthrough, non-linearity and common-mode output levels for LM13600, NE5517 and CA3280 were carried out. The effects of such errors on the overall performance and stability of multiple current systems when driving floating loads are considered.

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