PRL in sera of four prepubertal Guernsey bulls increased 233%, when light periods were lengthened gradually (0.38 h daily for 6 weeks) from 8 to 24 h, but maintenance of continuous illumination caused PRL to decrease within 1 week to concentrations not different from those observed during 8-h light, 16-h dark (8L:16D) photoperiods. To determine the effects of other photoperiods on serum PRL, eight Holstein bulls were exposed to 6 weeks of 8L:16D, followed immediately by 8 weeks of 16L:8D or 20L:4D. Basal PRL averaged 8 ng/ml at the end of the 8L:16D photoperiod. The first detectable increase in PRL occurred within 1 week after photoperiod was shifted from 8 to 16 or 20 h of light/day. Maximal concentrations of 67 and 56 ng PRL/ml serum (P > 0.10) were achieved after 5–8 weeks of exposure to 16L:8D or 20L:4D, respectively. To test effects of wavelength, eight Holstein bulls were exposed to 8L:16D photoperiods of cool-white fluorescent light (370–750 nm) for 15 weeks. Between weeks 5-10, four were exposed to red fluorescent light (550–750 nm) while four were subjected to blue fluorescent light (300–425 nm) for an additional 8 h. PRL increased (P < 0.01) from 10 ng–ml at the end of the 8L:16D photoperiod of cool-white light to 57 and 38 ng–ml when photoperiod consisted of 8 h of cool-white light plus 8 h of red or blue fluorescent light, respectively. Concentrations of PRL at the end of the 5 weeks of exposure to red or blue light were not significantly different from each other (P > 0.10). When red and blue lights were discontinued (bulls received 8 h of cool-white light daily), PRL decreased to concentrations similar to values initially observed during the first 5 weeks of 8L:16D cool-white light. Capacity of the anterior pituitary to release PRL after injection of a standard dose of TRH under different durations or wavelengths of light paralleled changes in basal concentrations of PRL. Light-induced changes in PRL in sera collected through cannulas were proportional (although lower) to concentrations in sera collected by venipuncture. We conclude that 16–20 h of light/day will sustain greater PRL in sera of prepubertal bulls than 8 or 24 h of light/day. Furthermore, broad spectrum wavelengths of light between 550–750 and 300–425 nm increased serum PRL of prepubertal bulls as effectively as light between 300–750 nm. © 1979 by The Endocrine Society.
