Male rabbits aged from 4 to 4·5 months were subjected to a 8L: 16D photoperiod for 4 weeks and were then split into two groups. One group of13 males was maintained at 8L: 16D (group 8). The other included 16 males and was submitted to 16L: 8D (group 16). Two successive ejaculates zvere collected and analysed once a week for 6 months. Animals from group 8 were sexually more active and ejaculated significantly larger volumes of semen than those from group 16. Performance of males from group 16 was better for all other measurements, whether these were qualitative (motility, percentage of live spermatozoa) or quantitative. Averages for the total number of spermatozoa and number of live spermatozoa per ejaculate were significantly greater in males submitted to 16L: 8D (509 and 408 v. 452 and 344 × 106 spermatozoa, respectively). However the testes of rabbits in group 8 had a significantly greater volume than those in group 16 (22·3 v. 17·4 cm3, P < 0·001). After the animals were slaughtered when 11 months of age, the volume and weight of the testes and the testis and epidydimis reserves did not differ significantly between groups. Under our experimental conditions, the sperm output of bucks exposed to 16 h of daylength was greater than that of bucks exposed to 8 h of daylength but in vivo and post-mortem measurements did not reflect this difference.