The effect of ethephon on the growth of leaves and stems of P. pratensis, Agrostis sp., and A. sativa was investigated. The length of the leaves of all species as well as that of the internodes of stems of vegetative tillers of Agrostis sp. and of stems of reproductive A. sativa plants was reduced. Epidermal cell length was unchanged or somewhat increased, indicating a reduction in cell number. The rate of cell elongation may be decreased, however. A reduction in cell length was observed in the internode of generative oat plants that just started to elongate at the time of treatment. In vegetative tillers of P.pratensis and A. sativa ethephon promoted growth of the internodes by increasing cell length. Cell number was little affected. GA3 also promoted internode growth, causing an increase in cell length as well as in cell number. Application of ethephon and GA3 together gave an additive effect on cell length but a reverse effect on cell number. Effects of ethephon were observed for 5 or more weeks after treatment. Ethylene production from plants of A sativa increased, but the extra production had already decreased by 90% after 11 d. No stimulation of the ethylene production by the plants other than from the breakdown of ethephon was observed.