The author studied in thirty Thoroughbred horses the influence of age and gender on daily rhythms of serum osteocalcin (OC). The animals were divided into two groups. Group A: six male and six female 2 years old; Group B: six male, six female and six geldings 6 years old. They were housed individually in box-like stalls under natural photoperiod and environmental conditions. Blood samples were collected every 3 hours over a 48 hour period. Statistical differences of serum OC concentration due to different gender were observed in both Groups A and B. Daily rhythms of serum OC concentration were observed only in Group B, with nocturnal acrophase. In females the acrophase was statistically postponed for 1 hour compared to male and gelding. Male and female showed a more robust daily rhythm than the geldings. The results showed that blood sampling for determination of serum OC should be strictly standardized with regard to the time of the day.