In this study, the effects of intramuscularly (i.m.)-injected GnRH at the time of insemination on serum progesterone levels and pregnancy rates were investigated. A total of 23 cows (20 Brown Swiss and 3 Holstein) age between 3-11 years were employed as study material. The animals were synchronised with PGF₂ alfa and blood samples were collected three times a week commencing on the day of oestrus. On the day following oestrus, all cows were inseminated by the bull and divided into two groups. Immediately after the insemination, the first group (n=12, treated group) received 100 mcg gonadorelin and the second group (n=11, control group) was i.m. injected with 2 ml sterile NaCl. Following these applications, collection of blood samples, three times a week,was continued for 21-22 days. The blood samples were centrifuged, sera separated and stored in a freezer until assayed. Serum progesterone levels were determined by RIA.

Duration of the oestrus cycle after insemination was found to be similar to that of nonpregnant animals in the gonadorelin-injected and control groups. In the comparison of post-insemination progesterone levels of pregnant animals, on the 10^th day progesterone levels were found to differ significantly between the treated and control groups. When the post-insemination progesterone levels of nonpregnant animals on the 17^th and 21^st days were compared, the differences between the treated and the control groups were found to be statistically significant. Seven (77.7 %) of nine gonadorelin-administered cows and five (45.4 %) of 11 control animals were recorded to be pregnant.

In conclusion, administration of 100 mcg gonadorelin at the time of the insemination did not significantly affect the serum progesterone concentrations, but increased the number of pregnant cows. Therefore, i.m. injection of 100 mcg gonadorelin soon after insemination may be used to increase the pregnancy rates in cows. However, the mechanism for the drug action remains to be further investigated.