Effect of pluviosity and sun shine on sperm production and seminal characteristics of the fish Brycon henni (Pisces: Characidae). In Colombia the fish Brycon henni is a protected endemic species. It inhabits water bodies in coffee producing areas (700-1900 ma.s.l.; 4 degrees 35'56" N - 74 degrees 04'51" W; 18-28 degrees C). Insufficient knowledge of its basic biology and behavior prevent the commercial culture of this promising fish. We studied the production and sperm physiology of captive males. Along a year 20 samples were taken from each of 10 males. The sample was obtained by abdominal cefalo-caudal massage and transported to the laboratory at 4 degrees C. Except for September and October (maximum rainfall), sperm was always obtained in at least 50% of the males. Color, osmolality and pH were similar in all the samples. Volume, concentration, viability, motility and activation time were variable: sunshine had a positive effect on volume (Spearman p < 0.05) and on sperm concentration (Spearman p < 0.05) while pluviosity had a negative effect on volume and viability. The proportion of ions (Na, K, Mg, Ca) was constant along the year; Na being 10-fold higher than K and 100-fold higher than Mg and Ca; however the absolute concentration of all ions was slightly higher in April and in July (with no apparent relation with the other variables analyzed).