SEASONAL reproductive rhythms of animals living in natural conditions 1-4 as well as domestic animals5-6 have been the subject of extensive research. Seasonal rhythms in experimental laboratory animals living in a constant laboratory environment, however, have been considered improbable. The first report on seasonal rhythms in laboratory animals was made in 1958 by Gunn and Gould7, who reported a seasonal cycle in the uptake of 65Zn by the dorsolateral prostate of the male rat, with elevations in February-March and June-July. Comments on the possibility of seasonality of androgenic hormones8,9 only emphasise that no study has been reported on the male laboratory rat to determine whether there are seasonal variations in plasma testosterone and luteinising hormone (LH) throughout the year. We began such a study in January 1973 and report data collected up to June 1974, which suggest a circannual rhythm in sex hormones in male laboratory rats living in what is apparently a constant environment.
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