A lock release mechanism is provided in a cassette holder for magnetic tape cassettes. The magnetic tape cassettes have a pivotal closure lid (35) pivotably secured to opposite edges of a casing for movement between a closed position, in which the front opening of the casing through which a magnetic tape can be extracted for loading onto a rotary head is sealed while not in use, and an open position, in which the closure lid is pivoted away from said opening to allow access to the tape while in use. The cassette (28) also has a sliding closure member (36) slidably mounted on one face of the casing and co-operating with the aforementioned pivotal closure lid (35). The sliding closure member (36) is held in its closed position while the pivotal closure lid (35) is In its closed position to cover a bottom spacing through which a device constituting part of a tape handling mechanism may be inserted, and in its open position while the pivotal closure lid (35) is in its open position to allow said device of the tape handling mechanism to engage the casing through the bottom spacing and extract the tape for loading onto the rotary head. The sliding closure member (36) is retained in its closed and open positions by means of a lock member (55). The lock release mechanism is provided in a cassette holder of the recording and reproducing apparatus and comprises a projection means (9) having a portion (9b) which releases the lock member (55) to allow movement of the sliding closure member (36) from the closed position to the open position when the magnetic tape cassette (28) is inserted into the cassette holder for loading and a portion (9a) which releases the lock member (55) to allow movement from the open position to the closed position when the magnetic tape cassette (28) is ejected.