Male Syrian hamsters exposed to short photoperiods of 6 h light/day (6L:18D) show regression of the testes within 12 weeks. Chromosome preparations of the meiotic stages (pachytene, metaphase I (MI) and metaphase II (MII)), testicular weights relative to body weights, sperm counts, seminiferous tubule diameter and histological appearance were examined at intervals during regression and subsequent recovery in a long photoperiod (14L:10D). The fall of testicular weight was associated with the decrease in tubule diameter. Spermatogenesis and sperm count were reduced rapidly and finally ceased after 10 weeks in short days. The numbers of MI and MII cells relative to 100 pachytene cells progressively decreased during the short-day treatment, although the ratio of MI:MII stayed constant whenever there was meiotic activity (except in the first week of the recovery phase). This suggests that an increasing proportion of pachytene cells did not progress to MI with increased time in short days, but cells which did reach MI progressed to MII in the same proportions as in the control testes. Meiosis ceased after 10 weeks in short days. Recovery in the long days was marked by a peak in the number of MI and MII cells/100 pachytene cells soon after the return to long days. This preceded the return (to control values) of the sperm count by 10 weeks. Initial recovery in the first 3 weeks was very rapid in all the determined values.