An adapter assembly (500, 700) for connecting a loading unit to a handle assembly, the adapter assembly comprising: an elongate body having a proximal portion and a distal portion (504, 704), the proximal portion being configured for operable engagement with an actuation assembly and the distal portion defining a longitudinal axis and being configured to operably receive a trocar assembly (520, 720); and a seal member (508, 708) disposed within the distal portion of the elongate body; and a retaining mechanism (530, 730) disposed adjacent the seal member and configured to releasably secure the trocar assembly within the distal portion of the elongate body, the retaining mechanism including: first and second retention members (540a, 540b, 740a, 740b), the first and second retention members pivotable between: a locked position in which the first and second retention members are positioned to be received within respective first and second openings (523a, 523b, 723a, 723b) in a trocar housing (522, 722) of the trocar assembly when the trocar assembly (520, 720) is received within the distal portion of the elongate body; and a release position in which the first and second retention members are positioned to be spaced from the respective first and second openings in the trocar housing when the trocar assembly is received within the distal portion of the elongate body, characterised in that engagement of the first and second retention members with the seal member when in the locked position frictionally retains the first and second retention members in the locked position. an elongate body having a proximal portion and a distal portion (504, 704), the proximal portion being configured for operable engagement with an actuation assembly and the distal portion defining a longitudinal axis and being configured to operably receive a trocar assembly (520, 720); and a seal member (508, 708) disposed within the distal portion of the elongate body; and a retaining mechanism (530, 730) disposed adjacent the seal member and configured to releasably secure the trocar assembly within the distal portion of the elongate body, the retaining mechanism including: first and second retention members (540a, 540b, 740a, 740b), the first and second retention members pivotable between: a locked position in which the first and second retention members are positioned to be received within respective first and second openings (523a, 523b, 723a, 723b) in a trocar housing (522, 722) of the trocar assembly when the trocar assembly (520, 720) is received within the distal portion of the elongate body; and a release position in which the first and second retention members are positioned to be spaced from the respective first and second openings in the trocar housing when the trocar assembly is received within the distal portion of the elongate body, characterised in that engagement of the first and second retention members with the seal member when in the locked position frictionally retains the first and second retention members in the locked position. a locked position in which the first and second retention members are positioned to be received within respective first and second openings (523a, 523b, 723a, 723b) in a trocar housing (522, 722) of the trocar assembly when the trocar assembly (520, 720) is received within the distal portion of the elongate body; and a release position in which the first and second retention members are positioned to be spaced from the respective first and second openings in the trocar housing when the trocar assembly is received within the distal portion of the elongate body, characterised in that engagement of the first and second retention members with the seal member when in the locked position frictionally retains the first and second retention members in the locked position.