The present study determined the effect of a GnRH and PGF2α treatment on proportion of cows detected in oestrus, the distribution of oestrous activity and subsequent fertility. The study also incorporated the use of ovarian ultrasound and milk progesterone assay to established a better overall assessment of treatments. Holstein-Friesian cows, averaging 9000-kg milk/lactation, and fed according to their requirements were used in the study. Cows were regularly cyclic; at least 60 days postpartum, had had two oestrous cycles and were clinically sound before being considered for either experiment. In Experiment 1, 274 cows were treated at random with an i.m. injection of 10 μg of GnRH (buserelin) followed 7 days later by 0.75 mg oftiaprost based on the ultrasound diagnosis of a corpus luteum. At that time and 3 days later, milk samples were collected and assayed for progesterone. Cows with progesterone concentrations >1 ng/ml were considered to have a corpus luteum, and luteolysis was assumed to have occurred when concentrations 3 days later were <0.8 ng/ml. In Experiment 2, 1431 cows were treated as above. The response to treatment was assessed by presence of a corpus luteum 7 d after GnRH treatment, oestrous detection rate, and distribution of heats and ultimately on conception rate. Conception rate was compared to untreated control cows inseminated by the same inseminator and using semen from the same sires. Artificial breeding was carried out after oestrous detection and pregnancy diagnosis was done by ultrasound. In Experiment 1, luteolysis was induced in 244/274 (89.1%) cows following PGF2α administration, with 198/214 (92.5%) been diagnosed correctly in heat and 16/214 (7.5%) been diagnosed in error based on progesterone profiles. Oestrous activity was most concentrated on day 3 (59.1%), with 20.7% and 16.7% of heats recorded on d 2 and d 4, respectively. When compared to untreated controls, GnRH-PGF2α treated cows had a better conception rate (56.6 vs. 39.5%, P<0.01). In Experiment 2, 1012/1431 (70.7%) had a CL 7 days after GnRH injection; 709/1012 (70.1%) cows were detected in oestrus, and 359/700 (51.3%) cows became pregnant after insemination compared to 182/434 (41.9%) of untreated controls (P<0.05). The utilisation of GnRH and PGF2α seems to be a useful synchronisation scheme based on AI after oestrous detection. However, variations in fertility after treatment demands a better understanding offactors affecting herd fertility under controlled schemes.