The occurrence of fasciculation potentials (FPs) was studied in healthy subjects aged 18-25. In 25 males and 25 females 3 intrinsic foot muscles, the tibialis anterior and the gastrocnemius muscles on both sides were monitored with surface electrodes for 2 min periods. Only potentials with a peak-to-peak amplitude of at least 50 microV were counted. The number of FPs per minute (FPs/min) was significantly higher in the abductor hallucis (AH) and significantly lower in the tibialis anterior as compared to all other muscles (P < 0.001). Men had significantly more FPs in the AH than women (P < 0.05). In all subjects FPs were found in at least 1 AH. Cooling of the foot did not influence the numbers of FPs/min in the foot muscles. To study diurnal variation, all 5 muscles on both sides were monitored 3 times/day on 10 different days in another 10 subjects (5 males, 5 females). Only in the tibialis anterior did the number of FPs never exceed 3/min. In the other muscles considerable fluctuations were found, especially in the AH, where more than 100 FPs/min were occasionally recorded. In the course of the day a significant (P = 0.05) decrease in FPs/min was found for the AH muscle. In 8 subjects there was a significant correlation between the numbers of FPs in the left and right AH during successive recordings. This indicates that an, as yet unknown, general factor determines the fluctuations in numbers of FPs.