The aim of the present study was to determine the interaction between substance P (SP) and arginine-vasopressin (AVP) on basal and LH-stimulated testosterone production by Leydig cells isolated from hamsters kept under long or short days (LD-hamsters, SD-hamsters, respectively). SP inhibited the testosterone production of Leydig cells, its effect being more pronounced in the case of LH-stimulated steroidogenesis in LD-hamsters. Similarly, the addition of AVP to the culture medium resulted in a diminution of basal, as well as LH-stimulated testosterone secretions. When Leydig cells were co-incubated with SP (10(-7) M) and AVP (10(-7) M), a strong inhibition of the testosterone production by 50-60% was established in LD-animals. However, even within the experimental circumstances in SD-hamsters, the modulation of testosterone production by SP and AVP was evident. The reported results suggest that there is an interference of two regulatory pathways, namely photoperiodic dependence and paracrine control of testicular steroidogenesis in hamsters.