Chronic and excessive egg laying is a frequent clinical syndrome in pet birds that can result in reproductive, nutritional, or behavioral pathology. Environmental manipulations to suppress ovarian activity are not always successful, and surgical intervention may involve serious risk. Tamoxifen, an estrogen blocker used to treat women with breast cancer, has been suggested as a chemical method of blocking the hen's reproductive cycle. In this study, 20 budgerigars (Melopsittacus undulatus) of various ages, presumed to be female but not actively laying eggs, were treated with tamoxifen for 38–46 weeks. Adverse effects analogous to those reported in humans were not observed; these included regurgitation, weight loss, loss of appetite, decreased activity, feather changes (alopecia), lameness or wing drooping (bone pain), abdominal distention, and radiographic changes in bone density. In budgerigars treated with tamoxifen, the packed cell volume, total serum calcium concentration, and total protein concentration remained relatively constant throughout the study. Leukopenia was the most significant adverse effect observed in treated birds. When treatment was discontinued, white blood cell counts returned to reference ranges in most birds. Antiestrogenic effects of tamoxifen were suggested by a change in cere color from white or brown to blue in birds during treatment. If tamoxifen is used in budgerigars, birds should be screened for subclinical infectious disease before treatment begins, and white blood cell counts should be monitored.