N-nitrosopiperidine (NPIP) is a potent esophageal carcinogen in rats whereas structurally similar N-nitrosopyrrolidine (NPYR) induces liver, but not esophageal tumors. NPIP is a possible causative agent for human esophageal cancer. Our goal is to explain mechanistically these differing carcinogenic activities in the esophagus. We hypothesize that differences in metabolic activation of these nitrosamines could be one factor accounting for their differing carcinogenicity. α-Hydroxylation is the key metabolic activation pathway leading to nitrosamine-induced carcinogenesis. In this study, we examined the α-hydroxylation rates of [3,4-3H]NPIP and [3,4-3H]NPYR by male F344 rat esophageal and liver microsomes. The major α-hydroxylation products of NPIP and NPYR, 2-hydroxytetrahydro-2H-pyran (2-OH-THP) and 2-hydroxytetrahydrofuran (2-OH-THF), respectively, were monitored by high performance liquid chromatography with radioflow detection. NPIP or NPYR (4μM) was incubated with varying concentrations of esophageal microsomes and co-factors. Microsomes converted NPIP to 2-OH-THP with a 40-fold higher velocity than NPYR to 2-OH-THF. Similar results were observed in studies with NPIP and NPYR at substrate concentrations between 4 and 100 μM. Kinetics of NPIP α-hydroxylation were biphasic; KM values were 312 ± 50 and 1600 ± 312 μM. Expressed cytochrome P450 2A3, found in low levels in rat esophagus, was a good catalyst of NPIP α-hydroxylation (KM = 61.6 ± 20.5 μM), but a poor catalyst of NPYR α-hydroxylation (Km = 1198 ± 308 μM). Cytochrome P450 2A3 may play a role in the preferential activation of NPIP observed in rat esophagus. Liver microsomes metabolized NPYR to 2-OH-THF (Vmax/KM = 3.23 pmol/min/mg/μM) as efficiently as NPIP to 2-OH-THP (Vmax/KM = 3.80-4.61 pmol/min/mg/μM). We conclude that rat esophageal microsomes activate NPIP but not NPYR whereas rat liver microsomes activate NPIP and NPYR. These results are consistent with previous findings that tissue-specific activation of nitrosamines contributes to tissue-specific tumor formation.