1. Growth and development of two inbred lines of tomato and their hybrid were followed with respect to increase in weight, increase in height, differences of branching pattern, total phosphorus content, total nitrogen content, total starch content, catalase activity of the stem tips, and total proteolytic activity of germinating seeds and seedlings. 2. The morphological data indicate an early established and subsequently maintained size advantage for the hybrid. 3. No significant differences in total nitrogen, total phosphorus, or starch content were recorded. 4. The hybrids appeared to have higher catalase activity in stem tips, and there definitely were differences in the pattern of development of total proteolytic activity. 5. The results suggest that in certain instances the investigation of the bases of heterosis should be confined to the early growth stages and should deal with differences in the development of the enzyme systems and their activity.