This study compared the adverse effects of formalin administered by two different methods. Formalin mixed with the feed (2.5, 5.0 and 10 ml/kg) was given to 10-week-old White Leghorn cockerels for a period of 8 weeks. Simultaneously in other groups, a 3% solution of formalin was administered into the crops (5, 10, 15 and 20 ml/bird/day). Total amount of formalin utilized during the experiment in the feed of individual bird of groups given 2.5, 5 and 10 ml/kg was 6.25, 3.9 and 1.6% higher than those administered 5, 10 and 20 ml of 3% formalin into crop, respectively. Body mass and feed intake in all feed-mixed groups and those given 5 and 10 ml formalin (3%) into the crop were not significantly different from control. Administration of 15 and 20 ml formalin (3%) into crop resulted in depression, delayed onset of crowing, significantly decreased feed intake, lower body mass, decreased mass and volume of testes, ulceration in crops, sloughing of mucosa and petechial haemorrhages in proventriculus. All the groups given formalin had significantly smaller diameters of seminiferous tubules. Kidneys of the birds administered formalin into crop exhibited pyknotic nuclei of epithelial cells in proximal tubules. Non-significant differences in different parameters and lesser degree of pathological changes in birds given formalin-mixed feed than their corresponding crop-administered groups suggested that formalin present in the feed had partially evaporated. Therefore, birds ingested less amount of formalin than that originally mixed in the feed.