In the present study, 2-(methylthio) pyrido [2, 3-d] pyri-midines having a variety of 4- alkylamino and 4-anilino side chains were prepared and their cytokinin agonistic and antagonistic activities were examined. Quantitative structure-activity analysis indicated that both series of compounds have a common mode of interaction with the receptor, and the maximum width of the N4 side chains, W,,, is an important factor in determining their ...