Abstract. Genetic and phenotypic parameters for age at first start (AFS) and performance traits were estimated for Polish Thoroughbred and Arab horses using restricted maximum likelihood (REML) with an animal model. The traits representing a horse’s racing performance were log of earnings (EA) and square root of number of starts (NS). Separate analyses per first racing season and whole career were carried out. The first racing season comprised 1 759 Thoroughbreds and 1 026 Arab horses. The racing results of whole racing career were available for 1 319 Thoroughbreds and 815 Arab horses. The model accounted for fixed effects of birth year, month of birth, sex and trainer. The average of AFS of Thoroughbreds was 933.2 days for first racing season and 935.1 for whole career. The same AFS was observed at first season and whole career (about 1 199 days) in Arab horses. Heritability estimates for AFS were rather low (0.15-0.21) . All genetic correlations for Arab horses between age and performance traits varied from moderate to high (from −0.25 to −0.85). For whole career of Thoroughbreds, the genetic correlations had opposite signs compared to those observed in Arabs. Genetic correlation between AFS and EA in first racing season was very low in Thoroughbreds (−0.07), whereas high between AFS and NS (−1); the corresponding values for Arab horses were −0.25 and −0.76. The opposite signs of phenotypic and genetic correlations (0.16 vs −1 and −0.24 vs 1 in first season and whole career, respectively) between AFS and NS in Thoroughbreds were observed. Phenotypic correlations between AFS and performance traits were close to zero for whole career while were rather low and moderate (−0.22 and −0.37) for first season in Arab horses. Delay of the first start of Thoroughbreds is favourable from the point of view of future racing performance in whole career and has no effect on earnings in the first racing season. Arab horses beginning their career at a young age are start frequently and earn more money then those beginning their career later.