In a study, aimed at comparing seasonal reproductive development of European fallow deer (Dama dama dama) with Mesopotamian (D. d. mesopotamico) x European F1 hybrids, five adult males of each genotype, which had been raised together since birth, were maintained as a bachelor group. Morphometric (body weight, neck circumference and testis diameter), endocrine (plasma testosterone concentrations) and seminal (ejaculate volume, spermatozoa per ejaculate and spermatozoa motility) parameters were recorded at fortnightly or monthly intervals for a 15-month period, and antler status was noted daily during the general periods of casting and velvet stripping. In addition, two bucks of each genotype were blood sampled via indwelling jugular catheters every 30 min for 24-h periods on five occasions (2-3 months intervals) during the year, and plasma was analysed for concentrations of testosterone and LH. Parameter profiles of the two genotypes were compared by global and time series ante-dependence covariance analysis to investigate overall profile similarity and the seasonal nature of any observed differences. Plasma hormone profiles from high-frequency blood sampling were subjected to PULSAR analysis to determine pulse frequency and amplitude. Throughout the study hybrid males were approximately 30% heavier than European males. However, both genotypes exhibited dramatic but parallel patterns of body weight change (global P = 0.054). Neck circumference was correlated with body weight throughout (P < 0.05), with similar regression slopes between the genotypes at any sampling time (P > 0.10). Covariance adjustment to a common initial body weight was performed to eliminate the effects of large body weight differences on muscle hypertrophy and regression. While profiles of corrected neck circumference were significantly different at the global level (P < 0.01), analysis by time revealed differences occurring only during the latter period of muscular regression in spring. However, profiles of other parameters, including testis diameter, plasma testosterone concentrations, spermatozoa per ejaculate and percentage motile spermatozoa, exhibited significant displacement between genotypes (global P < 0.05) evident as 2-4 weeks advancement in the sexual development (late summer/autumn) and quiescence (spring) phases for hybrid males relative to European males. Furthermore, mean dates of antler casting and velvet stripping were significantly earlier by 2-3 weeks for hybrid males than European males (P < 0.05). High frequency blood sampling revealed markedly seasonal patterns of secretion of testosterone and LH, with hybrid males exhibiting an apparent earlier onset of high-amplitude testosterone 'surges' in February (late summer) compared to those occurring in April (autumn) for European males. When viewed collectively, the data indicate strongly that the Mesopotamian influence is evident in the earlier attainment of sexual development and fertility in late summer and autumn, and earlier onset of sexual quiescence in spring. This is in accord with anecdotal information on earlier reproductive patterns in purebred Mesopotamian fallow deer.