The dependence of neurotransmitter release on calcium was evaluated in adrenergic terminals from mice that were acutely withdrawn from chronic morphine treatment (CMT). A two fold increase in the number of writhes in response to an i.p. injection of acetylcholine was induced in mice by CMT and subsequent withdrawal. A shift to the left in the relationship between the excitatory junction potential (e.j.p.) amplitude and extracellular calcium concentration ([Ca]o) was induced in vasa deferentia from CMT-withdrawn mice. A reduction in the degree of facilitation of transmitter release during a short low-frequency train of impulses and an increase in the amount of transmitter release during a high-frequency train of impulses was induced in vasa deferentia from CMT-withdrawn mice. The adaptive mechanism of the terminals to the sustained presence of morphine may involve an increase in the probability that the release sites will release transmitter either via increase in calcium influx or an increase in the affinity of calcium to the hypothetical X-receptor.