The study examines children's command of transitivity permutations in Hebrew, where a change in verb-argument syntax entails a change in verb-morphology. 30 children aged two, three and eight were required to produce existing and novel Hebrew verbs differing in transitivity. Younger children showed a good grasp of the syntax and semantics, but not the morphological marking of transitivity, three-year-olds did much better, and eight-year-olds produced mainly adult-like responses. Results were higher on existing verbs than on novel forms. Direction of change had little effect with existing verbs, but with novel verbs success was much higher in changing intransitive to transitive forms than the converse. Some alternations proved easier than others, e.g. intransitive activity verbs in the basic pa'al verb-pattern yielded more causative hif'il forms than intransitive inchoative verbs in the nif'al pattern. Findings throw light on the development of derivational morphology, item-based versus class-based learning, and the impact of lexical productivity and language-particular properties on acquisition.