1. The two well-known methods of estimating rates of irreversible disposal (R) of blood-borne substrates in vivo by isotope experiments involve estimating the specific radioactivity (S) of the substrate in blood either after single intravenous injection of labelled substrate or during its infusion at a constant rate. The value of R is calculated from the S-time curve, usually by assuming: (i) a metabolic steady state with respect to substrate, (ii) the passage of all substrate through the blood, and (iii) the absence of certain types of recycling via blood. 2. In a theoretical investigation we show how experiments can be performed and R calculated from analyses of blood when one or more of the above assumptions is unjustified, by using glucose, ketone bodies, plasma free fatty acids and proteins as examples. In general the methods require single injection procedures, with estimation of the total quantity of label in the substrate in blood and the substrate concentration instead of only S. Such values give estimates of R with standard errors even when only one blood specimen is taken from each of a group of animals, as is convenient when working with small animals or substrates in low concentration, and when the animals are in a non-steady state in which constant infusion procedures are invalid. 3. Similar methods give the fraction of label injected as one compound which passes through another (the isotopic yield). 4. The methods are not always applicable, and cannot be applied to plasma proteins in some pathological conditions. A questionnaire for assessing their applicability is given.

1. The plasma concentrations of glucose, lactate, amino acids, non-esterified fatty acids, glycerof, ketone bodies, ethanol, cortisol and insulin were measured in patients withii a few hours of injury and before treatment.' The severity of the injuries was assessed by the Injury Severity Score (ISS) method.2. Plasma lactate and glucose concentrations both rose significantly with increasing ISS.3. The concentrations of non-esterified fatty acids and glycerol were greater after moderate (ISS 7-12) than after minor (ISS 1-6) injuries. The glycerol concentrations were no higher and the non-esteritied fatty acid concentrations were lower after severe (ISS > 12) than after moderate injuries. The concentrations of total ketone bodies tended to follow those of non-esterified fatty acids and there was a highly significant correlation between them. 4. The total concentration of amino acids was not affected by the severity of injury and there were no systematic changes in the concentrations of individual ones. 5. Plasma insulin concentrations were very variable and not related to severity. A weak correlation with the plasma glucose concentration seen after minor and moderate injuries was lost in the severely injured. Correspondence: Professor H. B. Stoner, MRC Trauma Unit, Stopford Building, University of Manchester, Oxford Road, Manchester M13 9PT, U.K.6. The plasma cortisd concentration was positively related to ISS up to ISS 12 but negativeIy so in the severely injured.7. Factors such as age, sex and time after last meal were investigated. The most important factor modifying the response was intake of ethanol, which reduced the plasma concentrations of glucose, non-esterilied fatty acids and alanine and raised that of lactate as well as the hydroxybutyratel/LacetoacetateI ratio,