While it is often conjectured that elected politicians take lobbying jobs to cash in on their political experience, no quantitative evidence has been collected to investigate this proposition. I argue that legislators gauge their own career prospects by observing how successful their former colleagues -- who now work as lobbyists -- are. I document that when private sector career prospects improve, so does the probability that the average US senator leaves office to take a lobbying job. There is no effect immediately before a senator's pension scheme improves, and senators, who retire from working life after Congress or are elected to a safe seat, are unaffected by private sector career prospects. This indicates that senators react to opportunity costs associated with being in office. Finally, while the results suggest that certain ideological types are more attracted by private sector rewards than others, low turnover in the Senate makes it unlikely that the revolving door has changed the composition of the chamber.