A pharmaceutical and/or veterinary formulation comprising deslorelin and an excipient, the formulation being characterised in that, in vitro, it releases deslorelin into phosphate buffered saline, as hereinbefore described, at 37° C. at a rate of about 2-80 &mgr;g/day for at least 200 days. The formulation may be used for prevention of reproductive function, particularly in dogs and cats, and for the treatment, particularly in humans, of prostate and breast cancer and other diseases and conditions where suppression of testosterone or estradiol levels is beneficial.