A protocol has been developed which decreases the time for administration of N-nitrosopyrrolidine (NPYR) to male Syrian golden hamsters from 25 weeks to a single i.p. injection. Animals were divided into five groups: group I received two 0.5-mmol doses on alternate days; group II was given three 0.33-mmol doses on alternate days; group III received a single dose of 0.5 mmol; group IV was given a single dose of 0.25 mmol and group V served as a control and received saline. Preneoplastic and neoplastic changes in the upper respiratory tract and liver were observed in all carcinogen-treated groups. The number of animals with laryngeal and tracheal tumors in the NPYR-treated groups was dose-dependent. Groups I and II, respectively, had 21 of 26 (81%) and 18 of 24 (75%) animals with either laryngeal or tracheal tumors. Groups III and IV showed 4 of 12 (33%) and 3 of 13 (23%) hamsters with these tumors. No laryngeal or tracheal tumors were observed in control animals. These results indicate that a single dose of NPYR is sufficient to induce respiratory tract tumors in Syrian golden hamsters.