Drip-irrigated cotton often yields more than the conventional furrow-irrigated crop. The basis for such a yield increase is unclear. We conducted a 3-year study of upland cotton water relations with daily drip or biweekly furrow irrigation. All comparisons of plant water relations were made early in the biweekly irrigation cycle to assure ample soil moisture. In 1984, 1985, and 1986, drip-irrigated plants had a relatively constant midday leaf water potential during each season. The furrow-irrigated plants were similar both early and late in the season, but their water potential was considerably lower at or near cutout. Stomatal conductance (1984) and transpiration (1986) were also lower during that period. In 1986, plant hydraulic conductances were estimated as slopes of regressions relating single-leaf transpiration rates to leaf water potentials beginning predawn and ending at midday. Again, the hydraulic conductance was lowest at or near cutout. In 1984 and 1986, cutout in drip-irrigated plants was shorter and less severe than in furrow-irrigated plants, leading to more boll set per plant. In 1985, there was no advantage to drip irrigation. The data indicate that furrow-irrigated plants may have inadequate capacity to transport water from soil to tops when the assimilates are being partitioned mostly to the bolls (perhaps from root system senescence and declining total root length). Daily drip irrigation appears to overcome this limitation.