Although tubular aggregates (TAs) are nonspecific findings observed in various neuromuscular disorders, they are specifically recognizable in the muscles of patients with periodic paralysis. However, their function and significance are still unknown. We have previously reported that TAs were observed in normal male, but not in female mouse muscles. To clarify the effects of sex hormones on morphogenesis of TAs, the following studies were performed. In normal male mice (ICR/JCL, N = 102), TAs began to appear at 3-4 months, and were seen in all animals above 6-7 months of age. On the other hand, in normal female mice (N = 50), TAs were observed in only 2 mice, at 14 and 18 months of age. When mice were castrated (N = 69: male 34/female 35), TAs were not found in either males or females. The results suggested that there is a close relationship between TA formation and male hormone. Testosterone propionate (TP) was administered to both castrated and un-castrated mice (N = 167: male 84/female 83). TAs were not found in castrated, TP 10 mg/2M implanted mice (N = 22: male 12/female 10). When TP was increased to 20 mg/1M, TAs began to appear in both male and female, castrated and un-castrated mice at 3 months, and were observed in 100% of mice, except female un-castrated mice, older than 4-5 months of age. Accordingly, a fairly large amount of TP above a certain threshold and a long incubation time of at least 3 months is necessary to induce TAs in the skeletal muscle.(ABSTRACT TRUNCATED AT 250 WORDS)