Many female cats are spayed to prevent problems associated with calling and unwanted pregnancies. This study describes the safety and efficacy of an alternative approach, using an azagly-nafarelin containing implant (Gonazon) inserted subcutaneously in the neck of six treated queens for 3 years. These six queens together with six controls were permanently housed with vasectomized tom cats, and changes in progesterone concentrations were used to document the contraceptive efficacy of Gonazon. All six control queens ovulated regularly throughout the treatment period (3 years), as shown by regular changes in progesterone concentration. Sixteen ovulatory cycles were observed in each control throughout the study. In Gonazon treated queens, during the week following implant placement, two queens displayed a treatment-induced rise in progesterone concentration. Later on, all treated queens continuously displayed low progesterone concentrations until 3 years post-implant insertion, with the exception of a single isolated episode (at approximately 2.5 years of treatment), of follicular luteinization in two queens. In all queens, azagly-nafarelin concentrations peaked in the week following implant insertion remained high for 1 month and later decreased slowly. After 2.5 years of treatment, azagly-nafarelin concentrations were still greater than 150 pg/ml in 3/6 queens. During a 6-month long extension of the study (36-42 months post-treatment), all queens (treated and controls) were run with intact tom cats. None of them conceived. Following autopsy, ovarian weight and diameter of the uterine horns of 3/6 treated queens were shown to be similar to those of the controls. In conclusion, this study demonstrated that Gonazon efficiently prevented ovulation in queens (100%) for 3 years. Return to fertile heat was not observed towards the end of treatment. However, in half of the treated queens, reversibility of the treatment induced effects on the genital tract was demonstrated.