In this study the effects of using VPNs on the throughput of wireless LANs are investigated. To this end we have set up a test bed and based our observations on the actual measurements obtained from the experiments performed using this test bed. The wireless LAN we have used is an IEEE 802.11g network and we compared the throughput values obtained by employing IPSec in this network with the calculated ones. We performed the tests using periodically generated traffic with 1ms and 5 ms intervals between successive packets. We have also considered the case for random traffic generated according to exponential distribution. We present the variations of the throughput with respect to packet size for different input traffic patterns.