Vasoactive intestinal peptide (VIP) is a hypothalamic prolactin (PRL)- releasing factor in the turkey. The hypothesis in the present study was that active immunization of turkeys with VIP would neutralize endogenous VIP, decrease circulating PRL, and consequently prevent the expression of incubation behavior. Large white female turkeys were divided into three experimental groups comprising untreated controls, control turkeys immunized with keyhole limpet hemocyanin (KLH), and turkeys immunized with synthetic chicken VIP conjugate (KLH-cVIP). Each turkey received four injections at 4- wk intervals, starting on the day of photostimulation. The immune response, measured by the percentage binding of monoiodinated chicken VIP (cVIP) to plasma at a dilution of 1:1000, averaged 11.8 ± 2.5% during the reproductive life cycle. Immunization against KLH-cVIP prevented the normal increases of PRL that are associated with the photo-induced reproductive cycle. Over a 21- wk period beginning at photostimulation, KLH-cVIP-immunized birds exhibited a maximal plasma PRL level of 82.2 ± 29.5 ng/ml, compared to 367.7 ± 66.6 ng/ml and 227.5 ± 51.7 ng/ml for non- and KLH-immunized turkeys, respectively. The total number of nest visits per hen during the 147-day experimental period decreased from 320.0 ± 48.2 in the nonimmunized controls to 180.7 ± 53.7 and 149.4 ± 13.1 visits in KLH- and KLH-cVIP conjugate- immunized turkeys. Turkeys that showed an immune response to KLH-cVIP immunization did not exhibit incubation behavior, whereas 54% and 33% of non- and KLH-immunized hens incubated their eggs. The latter two groups laid 58.4 ± 7.2 eggs and 65.6 ± 9.9 eggs per hen, respectively, compared to 87.6 ± 3.1 eggs per hen for the KLH-cVIP-immunized turkeys. It is concluded that the increased egg production observed in KLH-cVIP-immunized turkeys is the result of failure to express incubation behavior due to lower PRL levels.