The pineal gland (PG) possesses receptor proteins capable of binding androgen with high affinity and specificity. However, the sensitivity of PG to exogenous testosterone (T) in term of biochemical constituents during different reproductive phases is still unknown. Hence, an attempt was made to study the effect of testosterone propionate (TP; 20 μg/animal/day; i.m. injection) on the biochemical constituents of PG i.e. protein, cholesterol, serotonin and plasma melatonin (MEL) level of the subtropical zone rodent, Funabulus pennanti during reproductive active (RAP) and inactive phases (RIP). During RAP, TP increased the plasma MEL level and prostate gland weight while, it had no effect on other biochemical constituents of PG, plasma T level and testes weight. It may be suggested that during the RAP, exogenous TP initiates the negative feed back mechanism at leydig cell level. Further, TP is known to affect directly the MEL synthesis by increasing the activity of PG. During RIP, TP decreased the PG weight, plasma MEL level, serotonin and protein content and increased the testes, accessory sex organ's weight and plasma T level. This decrease in PG activity may be due to the decrease in enzyme activity by TP which in turn inhibited the MEL synthesis and initiated the gonadal function.

Therefore, in this squirrel PG biochemical constituents presented a reproductive phase dependent variation modulated by T. Further, active PG of RIP appeared to be more sensitive to exogenous TP treatment in reducing the biochemical constituents and increasing MEL synthesis.