Androgen binding protein (ABP) was detected in both the testis and epididymis of golden hamsters exposed to a long photoperiod (16L:8D). The concentration of ABP in the testis rose from 0.1 pmol/g testis in 2-week-old animals to attain maximum values (3.9 pmol/g testis) at 6-7 weeks, then declined to adult values (1.8 +/- 0.4 pmol/g testis) after 10-11 weeks of age. In contrast, the ABP concentration of the caput epididymidis reached maximum values at 4-7 weeks of age (14 pmol/g tissue) and declined to adult values (4.8 +/- 1.5 pmol/g tissue) by 10-11 weeks of age. ABP content of the corpus epididymidis was maximal (1.0 pmol/g tissue) at 2 weeks of age and thereafter declined to below detectable levels by 10-11 weeks. No ABP could be detected in the cauda epididymidis from animals of any age examined. Hamster ABP analysed by steady-state polyacrylamide gel electrophoresis had a relative mobility (Rf) of 0.33 compared to 0.41 for rabbit ABP. Sucrose gradient analysis of hamster ABP indicated a sedimentation coefficient of about 4 S. The binding of [3H]5 alpha-dihydrotestosterone [( 3H]5 alpha-DHT) to hamster ABP was very rapid with equilibrium occurring within 10 min. The dissociation of [3H]5 alpha-DHT from hamster ABP was also rapid (t1/2 = 2.77 min). Saturation analysis of ABP from mature animals yielded an apparent dissociation constant of 6.4 nM and an ABP concentration of 1.2 +/- 0.2 pmol/mg protein. The binding of [3H]5 alpha-DHT to hamster ABP was inhibited by 5 alpha-DHT greater than testosterone greater than greater than greater than oestradiol greater than cyproterone acetate. Exposure of mature hamsters to a short photoperiod (8L:16D) for 3 weeks resulted in a 42% drop in epididymal ABP levels (10.3 to 4.3 pmol/g tissue). Epididymal ABP further declined so that after 15 weeks in a short photoperiod it was 4% (0.4 pmol/g) of initial values. Accompanying this decrease in epididymal ABP concentrations was a decline in the fertilizing ability of spermatozoa from the distal cauda. When hamsters were transferred from a short to a long photoperiod (16L:8D), epididymal ABP content returned to about 50% of control values within 3 weeks. However, the fertilizing ability of spermatozoa from the cauda epididymidis of these animals did not return to control values after a 9-week exposure to a stimulatory photoperiod.