Equilibrium predictions from the Perkins midlatitude spread F instability theory are tested by using nighttime incoherent scatter data obtained at the Arecibo Observatory. The field line integrated Pedersen conductivity is obtained by two different methods, one representing direct field line integration and the other using the equilibrium formula developed by Perkins. Of the four nights considered, agreement is good on three of the nights for times when an equilibrium is appropriate. During the post-sunset rise and the midnight collapse, when equilibrium does not exist, agreement is poor. Calculation of the east-west neutral winds required for instability growth times of 30 min indicate that east-west wind velocities would need to be comparable to the measured north-south winds. Shorter growth times would require higher east-west wind velocities.