AIM: To explore the distribution and metabolism of 131 I-gelatin microspheres ( 131 I-GMSs) in rabbits after direct injection into rabbits’ livers.METHODS: Twenty-eight healthy New Zealand rabbits were divided into seven groups,with four rabbits per group.Each rabbit’s hepatic lobes were directly injected with 41.336 ± 5.106 MBq 131 I-GMSs.Each day after 131 I-GMSs administration,4 rabbits were randomly selected,and 250 μL of serum was collected for γ count.Hepatic and thyroid functions were tested on days 1,4,8,16,24,32,48 and 64 after 131 I-GMSs administration.Single-photon emission computed tomography (SPECT) was taken for each group on days 0,1,4,8,16,24,32,48,64 after 131 I-GMSs administration.A group of rabbits were sacrificed respectively on days 1,4,16,24,32,48,64 after 131 I-GMSs administration.Their livers were taken out for histological examination.RESULTS: After 131 I-GMSs administration,the nuclide was collected in the hepatic area with microspheres.The radiation could be detected on day 48 after 131 IGMSs administration,and radiography could be seen in thyroid areas in SPECT on days 4,8,16 and 24.One day after 131 I-GMSs administration,the liver function was damaged but recovered 4 d later.Eight days after 131 I-GMSs administration,the levels of free triiodothyronine and free thyroxin were reduced,which restored to normal levels on day 16.Histological examination showed that the microspheres were degraded to different degrees at 24,32 and 48 d after 131 I-GMSs administration.The surrounding parts of injection points were in fibrous sheathing.No microspheres were detected in histological examination on day 64 after 131 I-GMSs administration.CONCLUSION: Direct in vivo injection of 131 I-GMSs is safe in rabbits.It may be a promising method for treatment of malignant tumors. AIM: To explore the distribution and metabolism of 131 I-gelatin microspheres (131 I-GMSs) in rabbits after direct injection into rabbits’ livers. METHODS: Twenty-eight healthy New Zealand rabbits were divided into seven groups, with four rabbits per group .Each rabbit’s hepatic lobes were directly injected with 41.336 ± 5.106 MBq 131 I-GMSs.Each day after 131 I-GMSs administration, 4 rabbits were randomly selected, and 250 μL of serum was collected for γ count.Hepatic and thyroid functions were tested on days 1,4,8,16,24,32,48 and 64 after 131 I-GMSs administration. Single-photon emission computed tomography (SPECT) was taken for each group on days 0,1,4,8,16, 24,32,48,64 after 131 I-GMSs administration. A group of rabbits were sacrificed respectively on days 1,4,16,24,32,48,64 after 131 I-GMSs administration. Their livers were taken for histological examination .RESULTS: After 131 I-GMSs administration, the nuclide was collected in the hepatic area with microspheres. radiation co uld be detected on day 48 after 131 IGMSs administration, and radiography could be seen in thyroid areas in SPECT on days 4, 8, 16 and 24. One day after 131 I-GMSs administration, the liver function was damaged but recovered 4 d later . Bright days after 131 I-GMSs administration, the levels of free triiodothyronine and free thyroxin were reduced, which restored to normal levels on day 16. Histological examination showed that the microspheres were degraded to different degrees at 24, 32 and 48 d after 131 I-GMSs administration. The surrounding parts of injection points were in fibrous sheathing. No microspheres were detected in histological examination on day 64 after 131 I-GMSs administration. CONCLUSION: Direct in vivo injection of 131 I-GMSs is safe in rabbits. It may be a promising method for treatment of malignant tumors.