Single doses of up to 0·4 mg/kg oestrone injected s.c. were found to interfere with pregnancy in the rat up to the 11th day post coitum (p.c.). As little as 0·02 mg/kg was effective during the period of tubal transport, while 0·1 mg/kg was necessary to prevent implantation after the ovum was free in the uterus. After implantation had taken place, 0·4 mg/kg injected on days 8, 9 or 10 destroyed some or all embryos or, if injected on days 10 or 11, caused delay in parturition. Delay in parturition was no longer reliably produced by the injection of 0·4 mg/kg on the 16th day of gestation.
A single dose of 4·0 mg/kg testosterone prevented implantation in 75% of rats if injected s.c. during days 5–8 p.c., and produced either foetal loss or delay in parturition in 69% of rats when injected on days 9–11 p.c. A single dose of 20 mg/kg testosterone was effective in all rats tested in preventing implantation if injected on days 1 and 5, producing foetal loss if injected on day 9 or delaying parturition if injected on days 11 and 16.
A single dose of 4·0 mg/kg androst-5-ene-3β,17β-diol prevented implantation or caused foetal loss in 93% of rats injected on days 5–11 p.c. This hormone was considerably more potent in preventing implantation and causing foetal loss than its androgenic potency as compared with testosterone would suggest. In contrast to testosterone, androst-5-ene-3β,17β-diol did not cause delay in parturition at doses up to 20 mg/kg.
The effects of two other steroids tested, androst-4-ene-3,17-dione and 17α-methyl-androst-5-ene-3β,17β-diol, on pregnant rats were equivalent to or less than their androgenic potency would suggest.
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