Seasonal changes in enzyme activities and some components of Satsuma mandarin and sweet lime were studied. Although the main acid in mature Satsuma mandarin fruit is citrate, malate was predominantly accumulated in the very early stage of fruit development. In sweet lime, malate was chiefly accumulated throughout fruit development. Juice vesicle tissue in Satsuma mandarin fruit developed in four distinctive stages. In the first stage, enzyme activities and the contents of protein and nucleic acid increased. The activity of phosphoenolpyruvate carboxylase increased most rapidly. Cell division was observed in the first half of this stage. In the second stage, acids accumulated remarkably but enzyme activities and RNA content did not change. In the third, maturation stage, the content of RNA increased again. In the fourth stage, the contents of citrate and RNA decreased, whereas the activity of NAD-dependent isocitrate dehydrogenase increased. Compared with climacteric fruit, no remarkable increase in the activity of NADP-dependent malic enzyme was observed in citrus fruit during maturation, while activities of citrate synthetase and malate dehydrogenase increased fourfold. Respiratory activity did not rise as prominently during that time.