(1) The daily C0₂ flux of above-ground and below-ground components of Panicum coloratum growing in the northern Chihuahuan desert was determined during a spring drought and following the autumn reproductive phase of growth. The objectives were (i) to determine the whole-plant carbon balance after a fifty-two day drought when photosynthetic capacity was severely repressed, and (ii) to determine the whole-plant carbon balance during favourable soil-water conditions prevalent after the autumn reproductive period. (2) Water stress severely repressed diurnal photosynthetic capacity during drought. The below-ground biomass, however, consisted of suberized roots with low respiratory activity. The net result was a positive whole-plant carbon gain of 2·6 mg C g dry wt ^-1 day ^-1.  (3) Precipitation, amounting to 1·12 cm, ended the fifty-two-day drought, and increased the whole-plant carbon gain approximately six-fold. (4) The mean root/shoot quotient was 1·9 during the spring drought and 0·6 in the autumn. Reproductive tissue accounted for 13-34% (mean= 22%) of the above-ground biomass in the autumn. (5) Following reproductive growth, when soil-moisture conditions were favourable, the root system comprised suberized roots, non suberized nodal roots and other nonsuberized roots. Although the last two root types had high respiration rates, their presence in low quantities and low soil temperatures resulted in below-ground respiration approximately equal to that during drought. The whole-plant carbon gain during September was 17·2 mg C g dry wt ^-1 day ^-1.