The exogenous application of carbon monoxide [CO] is a valuable strategy which enables study of the effects under different stress conditions. However, in this experimental model a true endogenous CO production by plants cannot be measured. In this work, so as to achieve an elevated sensitivity and to avoid invasive techniques, we quantify the endogenous CO production by tissues in salt-treated soybean plants through gas chromatography coupled to a reduction gas detector. This technique allows short and room temperature incubation of intact tissues and homogenates. We found that a 200. mM NaCl treatment induces total CO production in leaves and roots. The sensitivity of the technique offers no correlation between this increment and heme oxygenase [HO] activity measured as a function of CO production. We also found that untreated soybean plants continue to produce significant CO levels up to 7 days post planting, after which CO content decreases to a third and remains constant in the next days. However, HO activity does not change throughout these days. The data here reported shows that HO activity is not the main source of CO in soybean plants. We discuss alternative sources that could be implicated in this production. Taking our own results and data reported by other colleagues, we propose lipid peroxidation and ureide metabolism as potential sources of CO.