A generalized hyperreflexia was observed in Skylab 2 crewmembers during the immediate postflight clinical evaluations. Unfortunately, no provisions had been made to quantitate the observed phenomenon. The cause of the hyperreflexia was not readily apparent at the time of the observation. Reflex changes have been noted in several of the Russian cosmonauts after extended spaceflight. Prior to the launch of Skylab 3, it was decided to measure and record a single tendon reflex and, if possible, the associated muscle potential. Duration of the Achilles reflex immediately postflight showed a significant (p greater than 0.01) shortening for all three crewmen. The reflex duration exhibited further significant shortening from preflight values on the fourth day after recovery. At the 16th day measurement, there was a signficant lengthening of the reflex for crewmenber 2 and 3 while crewmember 1 showed some lengthening which was not significant. By 29 d postrecovery, the reflex time of crewmember 1 had returned to his preflight value while the other two crewmen continued to show a significant lengthening of the reflex, although both were showing trends toward their preflight values. The muscle potential intervals were shortened immediately postflight and remained shortened during the 29 d of postflight evaluation. Currently, work is in progress to correlate these findings with other observed anatomical, physical, and biochemical changes. Final conclusions and an in-depth statistical analysis will be presented at a later date.