Factors affecting the characteristics of corpora lutea (number, left/right ovary origin, weight, DNA and progesterone content) were studied in 73 healthy bitches divided into two classes of age (< or =2.5 vs. >2.5 years; mean+/-S.E.=3.6+/-0.3 years; range: 0.7-10 years), weight (< or =20 vs. >20 kg; mean+/-S.E.=16.2+/-1.2 kg; range: 5-45 kg), reproductive status (pregnancy vs. diestrous; pregnant bitches N=41 and diestrous bitches N=32), stage of luteal phase (20-40 vs. 41-55 days) and ovulation rate (< or =7 vs. >7). Two different assessments were performed: (a) comparison of luteal tissue characteristics and progesterone content between pregnant and diestrous bitches and (b) investigation of the effect of animal age, weight and ovulation rate on individual corpus luteum (CL) parameters. None of the luteal parameters differed between pregnant and diestrous bitches, even when the stage of the luteal phase was considered. Age and weight of the bitch significantly influenced luteal tissue characteristics: heavier bitches had more and heavier CLs (P<0.001) and carried more foetuses (P<0.01), while older bitches had a higher number of CLs (P<0.001). In pregnant animals, the rate of foetuses to Cls was 78.4%. Luteal progesterone content was significantly affected by the ovulation rate (P<0.01). A significant individual effect (P<0.0001) was present on all the parameters in the single CL, with the right ovary carrying a higher CL number (P<0.01), with greater DNA (P<0.01) and P4 content (P<0.001). CLs of younger bitches showed a diminished efficiency of P4 production (P4/mg, P4/DNA) with a significant effect of the interaction between age and reproductive condition of the bitch on DNA and progesterone content (P<0.0001). These findings indicate that animal weight and age have a major influence on the characteristics of canine corpora lutea.