Changes in responsiveness for the stinging reaction of honeybees fixed in a holder after receiving 3 electrical shocks delivered with 1 min interval, was registered and used as measurement for the effect of 2 microliter of different solutions injected. Every shock consisted of a train of pulses of 1 msec each, delivered for 2 sec at a frequency of 100 Hz. Injection of morphine-HCl (50 to 200 n-moles/bee) produced a dose dependent reduction of the honeybee stinging response to the electrical shocks. The morphine dose that produced a 50% inhibition of the response (D50) was 148 n-moles/bee (927 micrograms/g), i.e., a value far greater than that reported for vertebrates in behavioral test of analgesia. Naloxone 1.1 micrograms/g produces a significant reduction of morphine D50 effect and at 4-5 micrograms/g, a full disinhibition. Thus, whereas the D50 of morphine for honeybees is far greater than that for vertebrates, the doses of naloxone that antagonize morphine are similar for bees and vertebrates. Possible explanations of this difference are mentioned. Injections of met-enkephalin, leu-enkephalin, kyotorphin and (D-Ala2) methionine-enkephalinamide, given in doses of 200 n-moles/bee, an amount greater than that of the morphine D50, exhibited no effect on the stinging response.