Three mechanisms can explain second-order conditioning: (1) The second-order conditioned stimulus (CS2) could activate a representation of the first-order conditioned stimulus (CS1), thereby provoking the conditioned response (CR); The CS2 could enter into an excitatory association with either (2) the representation governing the CR, or (3) with a representation of the reinforcer evoked by the CS1. A series of experiments using second-order autoshaping with birds was performed to examine these possibilities. Following second-order autoshaping, birds' responding to the CS2 was found to be unaffected by extinction of the CS1, a result interpreted as showing mechanism (1) to be unimportant and implicating either one or both of the other mechanisms. The CS2 was also shown to provide the birds with specific sensory information about the reinforcer, a pattern of results uniquely predicted by mechanism (3). Implications for the understanding of second-order conditioning and for the SOP model (Wagner, 1981) of associative learning are discussed.