Improvements are implemented (Version 4) in a Computer-Based Respiratory Measurement System (CBRMS) identified as Version 3. The programming language has been changed from Pascal to C. A Gateway 2000 desktop computer with 486 DX2/50MHz CPU and a plug-in data I/O board (KEITHLEY METRABYTE/ASYST/DAC's DAS-HRES 16-bit Analog and Digital I/O board) replaces an HP 9836 system used in Version 3. The breath-by-breath system consists of a mass spectrometer for measuring fractional concentrations of oxygen and carbon dioxide and the accommodation of a turbine or pneumotachometer for measuring inspiratory and expiratory flows. The temperature of the inspiratory and expiratory gases can be monitored if temperature corrections are necessary for the flow measurement device. These signals are presented to the PC via the data acquisition module. To compare the two Versions, ten significant respiratory parameters were investigated and compared for physiological resting states and steady states obtained during an exercise forcing. Both graphical and statistical (analysis of variance, regression, and correlation) tests were carried out on the data. The results from the two versions compared well for all ten parameters. Also, no evidence of a statistically significant difference was found between the resting and steady-state results of the present CBRMS (Version 4) and the previous CBRMS (Version 3). This evidence suggests that Version 3 (Pascal) has been successfully converted to Version 4 (C). Implementation of the CBRMS in C on a PC has several advantages.(ABSTRACT TRUNCATED AT 250 WORDS)