Various ways for the opening of the rings of 2,2-dihydroxy-cyclohexylideneacetic acid lactone (Ⅱ) and 2-hydroxy-cyclohexylideneacetic acid lactone (Ⅲ) have been investigated. 5,6,7,8-Tetrahydrocinnolinone-(3) [Ⅳa, m.p. 188-189°, λ_max^EtOH 296 mμ (ε 2410), ν_max 3255(w), 3195(w), 1675(s), 1600(m), 1540(m) cm^-1] was formed from Ⅱ with hydrazine, and 2-keto-cis-cyclohexylideneacetic acid phenylhydrazide [Ⅵ, m.p. 136-137°, λ_max^EtOH 240mμ(ε 18,300)] from Ⅱ with phenylhydrazine. On heating only or on treatment with acid or alkali, Ⅵ underwent cyclization with a rearrangement into 2-phenyl-5,6,7,8-tetrahydrocinnolinone-(3) [ⅩⅥa, m.p. 121-122°, λ_max^EtOH 315 mμ (ε 4840), λ_max 1685(s), 1600(m)cm^-1]. The structure of ⅩⅥa has been established by elementary analysis, UV-and IR-absorption spectra, and the determination of active hydrogen. The mechanism of the above mentioned rearrangement has been discussed.