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1. To clarify the suitable application amounts of ureaform (Uf), a slowly available form of synthetic organic nitrogen, four plots which consisted of four levels of actual N (6, 8, 10, 12g per plant) were established. Furthermore, three plots whose plants received 8g of actual N with rape-seed cake (R), ammonium sulphate (A), and urea (U), respectively, were also made for comparison. Var. Earl′s Favourite for fall cultivation was used, and effects of these fertilizer treatments on the growth, the fruit weight and quality, and the concentration of various forms of nitrogen in the leaves of muskmelon were investigated.2. The fruit weight both in the plots of Uf-8 and Uf-10 was greater than that in the plot of Uf-12 as well as both in the plots of A and U. On the other hand, the external appearance of fruit in the plot of Uf-6 as well as in the plot of R, as measured by the net, and the days of post-ripening of the fruit, was superior to that in the plot of Uf-12 as well as both in the plots of A and U.3. With increasing the application amounts of ureaform from 6 to 10g per plant, the concentrations of NH4-N and NO3-N in the leaves, the electrical conductivity and NO3-N of the soil were increased.Judging from these results, especially, from the concentration of NO3-N in the leaves both in the plots of Uf-10 and Uf-12, the suitable application amounts of ureaform for muskmelon seemed to be 6 to 8g per plant as actual N.4. To clarify the trend of the mineralization of several nitrogen sources, the rates of ammonification and nitrification of ureaform, rape-seed cake, ammonium sulphate and urea incubated at the soil temperatures of 20°C and 30°C were measured.5. The ammonification rate of ureaform incubated at the soil temperature of 20°C increased up to 69% of the added nitrogen on the 28th day after incubation, and then gradually decreased down to 30% on the 63rd day after incubation. On the other hand, the nitrification rate of ureaform incubated at the same soil temperature increased up to 52% on the 63rd day after incubation.The ammonification rate of rape-seed cake incubated at the same soil temperature increased up to 60% on the 28th day after incubation, and then gradually decreased down to 27% on the 63rd day after incubation. On the other hand, the nitrification rate of rape-seed cake incubated at the same soil temperature increased up to 37% on the 63rd day after incubation.The trend of the nitrification rate of urea incubated at the same soil temperature was similar to that of ureaform.The nitrification rate of ammonium sulphate incubated at the same soil temperature only increased up to 10% on the 63rd day after incubation. This low nitrification rate seemed to be caused by the comparatively low soil pH brought by the added ammonium sulphate.6. Though the nitrification rates of ureaform and rape-seed cake incubated at the soil temperature of 30°C were much greater than those of the same nitrogen sources incubated at the soil temperature of 20°C, the nitrification rates of urea and ammonium sulphate incubated at the soil temperature of 30°C were slightly greater than those of the same nitrogen sources incubated at the soil temperature of 20°C.