This paper studies the effects of a conventional monetary policy shock in the USA during times of high financial stress. The analysis is carried out by introducing a smooth transition factor model where the transition between states (‘normal’ and high financial stress) depends on a financial conditions index. Employing a quarterly dataset over the period 1970:Q1 to 2008:Q4 containing 108 US macroeconomic and financial time series, I find that a monetary policy shock during periods of high financial stress has stronger and more persistent effects on macroeconomic variables such as output, consumption and investment than it has during ‘normal’ times. Differences in effects among the regimes seem to originate from nonlinearities in both components of the credit channel, i.e. the balance sheet channel and the bank‐lending channel. Copyright © 2016 John Wiley & Sons, Ltd.