The dipeptidase activity of the cotyledons of the squash (Cucurbita maxima Duch. ‘Hubbard’) increases during germination. Several herbicides inhibited this increase in various degrees. Those herbicides inhibiting the development of dipeptidase activity 70% or more were (2,4-dichlorophenoxy)-acetic acid (2,4-D), 3,6-dichloro-o-anisic acid (dicamba), (2,3,6-trichlorophenyl)acetic acid (fenac), and N-1-naphthylphthalmic acid (naptalam). Those inhibiting dipeptidase activity by 63 to 34% were 4-amino-3,5,6-trichloropicolinic acid (picloram), 2,6-dichlorobenzonitrile (dichlobenil), 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil), 3-amino-s-triazole (amitrole), 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), 7-oxabicyclo[2.2.1] heptane-2,3-dicarboxylic acid (endothall), and 5-bromo-3-sec-butyl-6-methyluracil (bromacil). Those causing only slight inhibition, 24 to 16%, were N,N-dimethyl-2,2-diphenylacetamide (diphenamid), isopropyl m-chlorocarbanilate (chlorpropham), and α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin); 3-(p-chlorophenyl)-1,1-dimethylurea (monuron) was essentially the same as the control. However, 2,2-dichloropropionic acid (dalapon), O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl)-benzenesulfonamide (bensulide), N,N-diallyl-2-chloroacetamide (CDAA), and 2-chloroallyl diethyldithiocarbamate (CDEC) were somewhat stimulatory. When compared with previously reported herbicide activity on the proteinase activity, those herbicides which inhibited dipeptidase activity did not necessarily affect proteinase activity to the same degree.