Gentamicin is an effective and economical drug used to control infectious diseases in poultry but is highly toxic and had slow clearance from the body.  This study aimed to report three cases of gentamicin toxicity in three White Leghorn (WLH) layer flocks in different poultry producing areas of Pakistan. In first case, gentamicin was injected @ 10 mg/kg body weight (BW) for seven times up to the age of 8-15 weeks in 9000 WLH layers.  In second case, gentamicin was injected @ 25 mg/kg BW four times in one week of 19-25 weeks old 7500 WLH layers. In third case, gentamicin was injected @ 33.33 mg/kg BW three times on alternate days in 23-29 weeks old 16000 WLH layers. Overall 56.85% mortality was recorded. Flock wise mortality was 8.69, 82.63 and 71.86%, respectively. Birds were dehydrated, emaciated and had prominent keel bone.  Clinical signs included decreased body weight, decreased feed intake, increased water intake and watery diarrhea. Necropsy revealed swollen kidneys bulging out from bony sockets. Petechial and echymotic hemorrhages were present on heart and skeletal muscles. Liver was enlarged with hemorrhagic streaks on its surface. Microscopically, hemorrhages and acute tubular necrosis was recorded in kidneys. Liver had hemorrhages, cellular infiltration and vacuolar (fatty) degeneration of hepatocytes. From the results, it can be concluded that overdosing and repeated administration of gentamicin is highly toxic to birds, therefore, be careful while treating the birds with this drug.

Key words: Gentamicin; Kidneys; Liver; Mortality; Toxicity; White Leghorn layer