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Department of Physiology, University of Sydney, NSW, Australia.

1. The effects of calcium on the secretion of quanta recorded from single varicosities on the surface of the mouse vas deferens has been determined. 2. If recordings were made from two adjacent varicosities and the [Ca2+]o increased from a low value (1 mM), then the increase in the mean quantal content of the endplate potential (m(e)) was almost entirely due to an increase in the binomial probability for secretion (pe). At higher [Ca2+]o there was an increasing tendency for the binomial parameter (ne) to increase from 1 to 2. When ne increased there was very little change in pe, indicating that the new release site recruited from the other varicosity has a relatively low probability for secretion. 3. If recordings were restricted to single varicosities and the [Ca2+]o increased in the range from 1 to 4 mM then the increases in m(e) were almost always due to an increase in pe. The gradient relating the log of m(e) to the log of [Ca2+]o for [Ca2+]o of 1.0-1.5 mM was in the range 3.2-5.4 (mean 4.2). 4. Test impulses gave a similar proportional increase in m(e) following a conditioning impulse at all varicosities from which recordings were made. 5. Facilitation of m(e) declined exponentially with an increase in the test-conditioning interval from 0.5 to 4 s. The time constant for this decline was about 6 s. 6. The results indicate that single release sites show a similar fourth power dependency on [Ca2+]o and facilitate to about the same degree following a conditioning impulse.