Twenty-four adult male Sprague-Dawley rats weighing 130-140 grams were used in this study. In experiment I. twelve intact adult sexu­ally active male rats.were sub-divided into two equal groups. In group A, six animals were injected with 0.2m1 saline 60 min. before experimenta­tion, while, in group 6, animals were injected with 10mg/kg. b. w mor­phine sulphate 60 min. before experimentation. In experiment II, twelve male rats were used to study the effect of acute morphine administra­tion in castrated male rat (castrated two weeks earlier). Animals were subdivided into two equal groups. In group A, animals were injected with 0.2 ml saline 60 min. before starting the experimentation, while, in group B, animals were injected with 10 mg/kg b. w. morphine sulphate 60 min. before experimentation. In both experiments, exploratory behaviour par­ameters (sniffing, rearing and investigating movement) were recorded for both latency and frequency, sexual behaviour parameters (mounting, intromission and ejaculation) were also recorded. At the end of 30 min­ute-behavioural test, all animals were sacrificed and trunk blood samples were collected. LH and FSH and prolactine were assayed in blood serum. Epididymal spermatozoa were collected and examined for spem concen­tration (million / ml) sperm viability (%), sperm motility (%) and abnor­mal forms (%). The obtained results indicated that, in experiment I, acute mor­phine administration resulted in a non-significant (P<0.05) changes in all of the exploratory behaviour, inhibition in all parameters of sexual beha­viour, significant (P<0.01) decrease in LH and FSH, and significant (P<0.01) increase in prolactin level. There was a significant (P<0.01) de­crease in sperm viability and motility, while, there were non-significant changes in sperm concentration and abnormal forms. In experiment acute morphine administration in castrated male rats resulted in a non­significant change on the exploratory behaviour; a complete inhibition of the sexual behaviour, and non-significant change in LH, FSH and prolactin level. It is concluded that, acute morphine administration affects signifi­cantly male reproduction, while, castration does not enhance the effect of morphine administration.