The introduction of a pipette or needle into the blastocoele has been used for measurement of the potential difference across the trophoblast (Gamow & Daniel, 1970; Cross, 1971), for aspiration of fluid (Lutwak-Mann, Boursnell & Bennett, 1960; Smith, 1970), for removal of a portion of the inner cell mass (Lin, 1969), for injection of teratogens (Lin & Monie, 1973) and for injection of cells into the blastocyst cavity (Gardner, 1968; Moustafa & Brinster (1972). Despite temporary distortion following manipulation, the blastocyst exhibits a remarkable ability to regain its original form. Capacity for normal development is also retained in most cases. Rabbit blastocysts which contracted after microsurgery seldom implanted unless they were allowed to re-expand in culture for 3½ to 9 hr before transfer (Gardner & Edwards, 1968). In contrast to conditions in the rabbit, work in our laboratory has shown that transfer of the contracted mouse blastocyst does not limit