A series of experiments were carried out in 1954 and 1955 designed to investigate the effects of MCPA and 2,4‐D applied at different stages of growth to grasses grown for seed. Single, spaced plants were sprayed at the rate of 1·5 lb. a.e. per acre with 2,4‐D amine in 1954 and with 2,4‐D amine and ester and MCPA ester in 1955. The development of the grass growing point was used as a criterion of the developmental stages at which the plants were sprayed.Some treatments caused abnormal types of growth. S.23 perennial ryegrass and S.143 cocksfoot developed ear malformations, S.53 meadow fescue had leaf and ear malformations, while S.51 timothy showed leaf, stem and ear abnormalities.Leaf malformations developed only after spraying when the growing points showed vegetative primordia while ear abnormalities usually appeared after treatment at most early stages of development. Stem malformation in timothy occurred after spraying at all stages in 1954, but only at the vegetative stages in 1955.The percentage of abnormal ears in a random sample of tillers in all but one case fell as the growing point developed and most damage occurred after treatment when the apex was at the vegetative stage. Normally, treatment when glume ridges were visible on the apex produced no significant difference between the number of abnormal ears in the treated and untreated plots.The results suggest that the chemicals act on that part of the apex which is just beginning its development but is not yet visible as a protuberance on the apex.The type of abnormalities occurring in grasses as a result of spraying with growth regulating weedkillers is similar to those found in cereals after similar treatment.Provided that the chemicals are applied in the 4 to 5 weeks prior to ear emergence the amount of malformation occurring in the grasses will be