IN clinical biochemistry laboratories, the importance of quantitative enzyme assays and enzyme-linked analyses steadily increases, and the merits of kinetic methods for monitoring these reactions are well recognised [1 ]. Reviews of the kinetic aspects of analytical chemistry [2] and of the applications of enzymes in analytical chemistry [3] indicate the need for absorptiometers which are able to monitor reactions continually over increasingly long periods of time. The performance of current instrumentation, however, is limited [4]; either the time during which reactions are followed must be limited in order to maintain an acceptable sample throughput, or the sample throughput restricted in order to obtain suitable reaction times.This paper describes the design of an analytical system which overcomes these two limitations. The system offers fast sample throughput, together with long observation times and good data resolution. The basic design principle is the sequential monitoring of a large number of cuvettes in a circular array, by means of a single multiplexed absorptiometer. The yield of data is determined by the time taken for the monitoring system to return to an individual cuvette, i.e. the scan rate and the total residence time of a sample in the system. The residence time of asample is in turn determined by the total number of cuvettes and the specimen throughput, corresponding to the frequency with which a new sample is introduced. The actual values of these parameters can be determined when a particular instrument system is configured. Thus, for a practicable design of instrument, if there were an array of 100 cuvettes, and a new sample were to be introduced every 24 seconds, then the maximum possible observation time could be 100 x 24 seconds 40 minutes, with a specimen throughput of (60 x 60)/24 150 per hour. The scan rate would be totally independent of the two parameters, so that if, for example, an individual cuvette were monitored at 2 second intervals, then in a period of 40 minutes, the sample would yield