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Testosterone production in testis tissue taken from rams that had been scrotally insulated for 28 days and from control animals was compared in the presence and absence of pregnenolone and LH. The absolute content of testosterone, but not its concentration was reduced in scrotally insulated rams. Tissue from treated animals was able to synthesize testosterone from exogenous pregnenolone as effectively as tissue from control rams, whilst LH and pregnenolone together had little additive effect. The addition of LH alone stimulated greater testosterone production by testis tissue from treated rams than from control testes. The addition of pregnenolone was followed by a rise in testosterone production within 1 h of incubation, whilst the LH stimulation of testosterone production was not evident until after 4 h of incubation. This suggests that the major action of LH in this system may be to simulate the conversion of pregnenolone precursors to pregnenolone. It is suggested that an accumulation of pregnenolone precursors occurs in the scrotally insulated testes and that these may be mobilized and cleaved after incubation with LH in vitro.

Three stallions were bled each hour for 25 h at 28-day intervals throughout 1 year. Testosterone levels were pulsatile. Pulse frequency and pulse amplitude were higher in the summer months than at other times (P less than 0.01). The number of testosterone pulses also varied throughout the day, with the greatest frequency occurring in the afternoon (14.00-17.00 h) and at night (22.00-01.00 h). Mean testosterone levels were highest in the summer (P less than 0.01) but showed a secondary, smaller increase in the autumn. Semen characteristics were assessed from measurements of 222 stallions. Semen volume was greatest in the summer (P less than 0.01) but both the concentration (P less than 0.01) and the total number of spermatozoa per ejaculate (P less than 0.05) were highest in the autumn. These results show that the highest testosterone concentrations in peripheral plasma are not necessarily associated temporally with optimum semen quality.

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