Eighteen pre-pubertal Nili-Ravi buffalo heifers (Bubalus bubalis) with mean (± S.E.) age 22±0.79 months and weight 290±4.62 Kg, respectively were randomly divided into PG (progesterone + GnRH), PN (progesterone + normal saline) and C (control) groups. Heifers in PG and PN groups were pre-treated with progesterone (Crestar implants) for 10 days, then assigned randomly to: PC) received pulses of GnRH (n=6;5 pg/2 h for 72 h), PN) received normal saline (n=6) in the same manner as PG group and C) served as controls (n=6). Blood samples were collected on the days 0(first day 1st of experiment), 3, 6 and 10 of progesterone pre-treatment and on days 11, 12 and 13 (during GnRH/normal saline treatment) and then 3 samples per week for 3 weeks.

In PG and PN groups, 100% (n=6) and 33.33% (n=2) heifers ovulated and exhibited oestrus. Behavioural signs of oestrus i.e. bellowing, frequent urination, position of tail, licking, appetite and isolation from herd were completely absent at oestrus, while mounting, allow mounting and acceptance of male were observed in 12.5%(n=l), 12.5% (n=1) and 87.5% (n=7) of heifers.

Changes in internal genitalia like ovulation from ovary, contraction of uterus/uterine horns and relaxed or open cervix were observed in 100% of heifers. Changes in external genitalia i.e. swollen vulva and hyperaeniia of vagina were observed in 100 and 87.5% of heifers, while 62.5% and 37.5% of helfers showed free flowing and by squeezing vaginal mucous discharge. The colour of the mucous was transparent, opaque and whitish in 37.5, 25.0 and 37.5% heifers. Consistency of the mucous was thin, viscous and thick in 25, 25 and 50% of the heifers, respectively.

It was observed that all (100%) buffalo heifers initiated the oestrous activity during evening hours and 75% and 25% heifers ended their oestruses during night time and day hours.

The overall mean (± S.E..) length of oestrus was 22±3.63 h (range 11 to 40 h). CL was palpable in all (100%) buffalo heifers on day 12 of the oestrous cycle which ovulated. The overall non- return rates were 25% (2/8).

Crestar implants (progesterone pretreatment) were not effective in heifers in maintaining the luteal-phase levels of progesterone in both PG and PN groups of buffalo heifers. Similarly, Plasma progesterone concentrations remained basal during pulsatile delivery of GnRH.

In GnRH-treated heifers, the ovulation was not followed by luteal-phase in 66.66% heifers, while only 33.33% heifers exhibited luteal-phase in which plasma progesterone concentrations rose (>l.Ong/ml) on days 9, 12 and 15 of the cycle.