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One purpose of this study was to investigate the initiation of pregnancy in virgin and multiparous female golden hamsters. A second purpose was to investigate the physiological significance of long intromissions (10-30 s of intravaginal thrusting). Virgin females required significantly less copulatory stimulation to induce pregnancy and maximize litter size. Animals receiving only long intromissions showed a 100% pseudopregnancy rate, a result indicating that long intromissions can initiate the neuroendocrine response necessary to maintain pregnancy. When the male delivered only one or a few ejaculations, the inclusion of long intromissions significantly increased the percentage of both virgin and multiparous females that became pregnant. Multiparous females were employed to test the fertilizing capacity of successive ejaculates from rested males. Order of ejaculations did not affect the proportion of females that delivered litters. However, litter size of females receiving the 1st or 5th ejaculation was significantly larger than that of females receiving the 10th or 15th ejaculation. Multiple-male matings did not augment pregnancy rates or litter sizes compared with matings with a single, rested male.Copulation serves multiple functions in addition to the placement of sperm in the appropriate region of the female reproductive tract. In the laboratory rat a single ejaculation is sufficient to initiate pregnancy and delivery of a normal-sized litter; however, this occurs with a high probability only when six or more intromissions accompany the ejaculation (Adler, 1969;Wilson, Adler, & LeBoeuf, 1965). Following ejaculation the male rat shows a refractory period of 5-11 min in which no copulation occurs. If the female receives further copulatory stimulation (intromissions) during the first 4 min of this refractory period, sperm counts in the uterus are significantly decreased as also is the number of implantation sites (Adler & Zoloth, 1970). In a further examination of these phenomena in the rat, Chester and Zucker (1970) showed that while three intromissions were suffi-This work was supported by Grant BNS-8300892 from the National Science Foundation.We thank N. T. Adler and J. P. Toner for their valuable comments on an earlier version of the manuscript.Requests for reprints should be sent to U.

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