The vegetative and reproductive growth of tropical forest plants often occurs in short bursts separated by longer quiescent periods. Seasonal changes in moisture availability may be a cause and, if so, may cue growth in a proximate sense or limit growth in an ultimate sense. Two 2.25—ha plots of mature forest were irrigated for three consecutive dry seasons on Barro Colorado Island, Panama, to discriminate among the various possibilities. The responses of three Piper species and four Psychotria species are reported. Irrigation disrupted the timing of leaf flush for all seven species. Irrigation increased annual leaf production for just one species, however, and had no effect on annual inflorescence production for the Psychotria nor on stem radial growth. These responses are consistent with the hypothesis that water stress constrains the timing of growth while only rarely limiting total annual growth. However, leaf turgor potentials observed among control plants may not have been sufficiently low to constrain leaf and inflorescence production, and other effects of irrigation cannot be discounted. Possible phenological cues that were affected by irrigation include soil moisture content, air temperature, relative humidity, and the timing of nutrient inputs from decaying litter. There was no evidence that a "water stress requirement" must be met during the dry season before wet season budbreak. The gradual decay in synchrony for leaf production for all four Psychotria species suggests that endogenous rhythms contribute to growth periodicities in the genus.