In a metabolic study of [1-(14)C]geranylgeranial involving rat thymocytes, the radioactivity was mainly incorporated into two metabolites, Z1 and Z2, the latter moving slower than the former on a silica-gel thin-layer plate. The time course of Z1 and Z2 formation superficially suggested a precursor-product relationship between Z1 and Z2. The two metabolites were chemically converted to their methyl esters on treatment with trimethylsilyl diazomethane. Z1 was cochromatographed with E,E,E-geranylgeranoic acid (GGA). Z2 was prepared in a large quantity from geranylgeranial using thymocytes, and purified by TLC followed by ESI (negative ion mode) or EI mass-spectrometry. The observation of a negative ion at m/z 305 on ESI and a molecular ion at m/z 306 (C(20)H(34)O(2)) with fragments similar to GGA on EI implied that Z2 was dihydroGGA, which has been detected in the urine and serum of patients with Refsum disease. The EI mass spectrum of (R)-2,3-dihydroGGA was identical to that of Z2. The diastereomeric amide synthesized from metabolite Z2 with (R)-1-(1-naphtyl)ethylamine was cochromatographed with (R acid, R) amide, not with (S acid, R) amide, which were similarly synthesized from (R)- and (S)-2,3-dihydroGGAs, respectively. In another metabolic study on [1-(14)C]geranylgeraniol (GGOH), the radioactivity was similarly incorporated into a metabolite corresponding to (R)-2,3-dihydroGGA. (R)-2,3-DihydroGGA induced DNA ladder formation with a maximum at 15 mciroM in thymocytes. However, 2,3-dihydrofarnesoic acid did not induce it at all.