Prostaglandins were described as potent mediators in the auto/paracrine regulation of reproduction. The canine Corpus luteum (Cl) is in part independent of gonadotrop support in the first third of diestrus. Analog to other species, it was postulated, that locally produced prostaglandins act via auto/paracrine mechanisms during this phase. Earlier experiments showed, that both cyclooxygenase 2 (COX2) and prostaglandin E2 (PGE2) -synthase (PGES) are strongly expressed in the early Cl-phase. To support the hypothesis of an auto/paracrine effect of PGE2, canine luteal cells were isolated in the early luteal phase and stimulated with PGE2. An upregulated StAR-promotor activity, an increased StAR-protein expression and an enhanced synthesis of P4 could be shown. In contrast, the expression of cytochrome P450scc and 3! hydroxysteroid dehydrogenase/$-5/$-4 isomerase (3!HSD) remained unaffected. Additionally, the expression of the prostaglandin transporter (PGT) in Cl of pregnant and nonpregnant dogs was assessed and revealed a time-dependent expression with strongest signals in the early luteal phase. Consequently, PGE2 proved to be a steroidogenic factor in luteal cells isolated during the early Cl-phase, while the expression pattern of PGT supports the hypothesis of prostaglandins acting via auto/paracrine mechanisms in this period.